Well unfortunately, she is probably going through a very stressful time right now. As you may/may not know, Saturn and Pluto - two very intense planets - are transiting Capricorn at around 24 degrees. So they are right on her ascendant now. Usually when either Saturn or Pluto crosses from the 12th house to the first, it signifies a very dramatic change in life and in how the world sees you. So she may feel very serious and focused, like she feels a strong pressure to present herself in a certain way to the world. It can unfortunately be a very stressful time, especially for a 10 year old who, since she's young, doesn't know exactly who she is yet. Since she has a Capricorn Moon, the Saturn/Pluto influence may make her feel like she needs to grow up fast, and that she doesn't have time to feel like a kid. Or, she may WANT to grow up fast and do more teenage stuff, like talk to boys more or even take on more leadership roles in class/school/extracurriculars. In any case, she may really be feeling the pressure to assert her own identity as someone competent.
So probably the best thing you can do is to be reassuring and helpful for her, to be emotionally supportive and nonjudgmental. Because the Capricorn Moon and Ascendant means she is already probably very hard on herself, and needs emotional support.
2020-2030 will probably be a very dramatic time for her, and her physical appearance may change considerably, since when Saturn and Pluto transit the first house (the house of appearances), a huge overhaul in the appearance usually takes place. It may be that she starts wearing darker or more serious/dramatic clothing, or maybe gives off a darker/deeper vibe or attitude. She may want to impress on others that she wants to be taken seriously, more like an adult and not just treated 'like a little kid' anymore. This may actually be a good thing for her parents, because she may enjoy taking on chores and responsibilities and thus earning her own independence. But in any case, Saturn/Pluto transiting the first house is often so dramatic that if you compare how she looks when she is in her 20s, to her childhood pictures, they may not even seem like the same person. This is especially true for a Capricorn Moon and Rising - the Moon is very important in a woman's chart because it represents her basic feminine self and feeling; and Capricorn is ruled by Saturn, so Saturn will really impress itself on a person who has Capricorn influence in the first house. So her bone structure in particular may really dramatically change - possibly becoming stronger, and the Aquarius influence may make her taller or lankier. Her facial bones or teeth especially may dramatically change.
Since Water signs are on the cusps of the body houses (2nd/6th), it may be that she has dramatic weight gain/loss issues as she grows. Her weight can also easily be influenced by her moods, and particularly puberty, which involves the sexual hormones. This is because Uranus in her 2nd house is square Pluto (a sexual planet), and is opposite Saturn in the 8th house (a sexual house). So she may have a very unique (Uranus) approach to sexuality or an otherwise interesting or altered sexual development. She may be prone to sexually experimenting or indulging in taboo sexuality as a teenager/adult, or even being sexually impulsive, so be wary of that and try and guide her to make informed decisions. Since Uranus is in 2nd house, she may also like tattoos or other unique ways to distinguish herself - i.e. clothing, a unique style, etc. But with Pisces on the 2nd house cusp, there is a chance that she gets tattoos or clothing on an emotional whim, and may regret them later.
Pluto in 12th house square Saturn in 8th house suggests potential sexual wounds or sexual abuse issues - either in this life or in past ones, that she may be forced to deal with as she ages. But with Pluto in 12th, these may be buried in her subconscious and may not easily make themselves known - so she may find herself acting out, acting addictively or compulsively with issues, until she is able to either meditate and heal herself or see a therapist who can help her. Capricorn in the 12th house, and Pluto there as well, suggests a person who likes to 'keep a tight lid' on their subconscious problems and neuroses and who would rather brush issues under the carpet, to either not acknowledge them or pretend like they don't exist. But the thing is that once Pluto transits through the first house, those problems will apparently 'explode' onto the surface and the person will be forced to deal with them somehow. So it's good if she has a supportive ear and guide to be there for her. Moon conjunct Pluto in 12th, also square Saturn, suggests that her mother or some authoritative female figure is related to her sexual pain and suffering. There may be a feeling of being too emotionally cold and judged, not feeling worthy of being loved or nourished. She may harbor serious resentment or anger towards the mother or her parents in general. The good thing is that if she can work out the wounds patiently and with effort, she can end up becoming an excellent and conscientious parent herself in her later life.
Another very important thing is that she has the asteroid of deep wounds, Chiron, in Aquarius, conjunct her Sun, Neptune, and Venus. This means that she likes to be (and sees herself as) very compassionate, friendly, and sensitive towards others (Venus and Neptune are very loving, gentle and compassionate, especially when together). Since Neptune is often considered a higher octave of Venus, and both are related to creativity, she has a very, very powerful creative energy and could be something of a mental genius, and would make an excellent artist. So please encourage her creativity throughout her life in whatever way she likes.
But the issue is that Chiron here, conjunct all these planets, also gives her a serious wound and potentially a lack of confidence here. Chiron in the first house means there is a wound to her sense of self and how she presents herself to the world. Perhaps she was bullied in a past life, or even in this one, for being too awkward and not really knowing how to connect with others. Aquarians can sometimes be very awkward and can live in their heads, and they may feel like they are interacting well with others, but they can be emotionally closed off (especially with Moon in Capricorn), and so they don't really understand how to relate well with others at their own level. So their interactions can come off as either forced or too unsure of themselves. Venus and Neptune are both very accomodating planets, not very forceful, so your sister may try to 'adapt' herself to others too much and may not be as secure in her individuality (Sun) as she likes.
So Saturn transiting her first house will almost force her to learn the lessons of how to relate properly with others. This is especially true since her natal Saturn is in Libra, which is the sign of relating. The good thing is that Saturn is exalted in Libra, so it does very well there - although it can also mean that a person may have a lot of hiccups and lessons to learn when it comes to interacting with others. So your sister may feel very weird and awkward around her close friends, or may have a hard time making any at first. She may experience painful social rejections and may lose confidence (Capricorn rising/moon can be very self-judgemental) because of this. This is especially painful because the teenage years are the puberty years, where dramatic changes happen sexually - Pluto, the sexual planet, is conjunct her Moon (emotions) which means many of her sexual body changes and hormones can really affect her feelings dramatically. She may get very intense mood swings, and in the 12th house, they may feel uncontrollable or she may not know where they come from. This is where she needs to learn to consciously pay attention to her feelings - because the 12th house signifies what has been buried in the past, so it seems that in past lives she liked 'stuffed down' a lot of her feelings and chose to not focus on them; now in this life, she will be forced to feel them (probably painfully) and deal with those feelings on a conscious level. Moon in Capricorn also can make one feel painfully not up to the mark or aware of one's shortcomings, so she may really need emotional reinforcement and help at times.
This is where you, an older sibling or someone she can trust - needs to be there for her and help her emotionally, to reinforce her confidence and to let her know that she is strong enough to handle emotions. Probably a key lesson of an Aquarius Sun/Capricorn Moon is to learn how to be more emotionally honest and non-judgmental about oneself. They tend to live too much in their minds and not in their hearts/feelings, which can lead to severe mental/emotional disorders later in life, especially if they cannot relate to others well.
Since Saturn involves structure, she may actually thrive and learn her lessons about relating to people in a more structured environment. So if she joined a group or extracurricular team, or a sports group, girl scouts, etc., they have structured environments and so she may feel better interacting with others in those arenas. Since Saturn and Pluto are transiting her first house, her peers and others may also see her as a leader and strong, and may be more receptive to listening to her leadership commands. But she will have to learn to not be too bossy or assertive, or to make up for any feelings of weakness by coming on too strong. Again, she will have to learn the lesson of balance, which is Saturn in Libra, by being patient with herself and others, and letting herself be emotionally vulnerable enough to relate to others' emotional needs (which is what Cancer- the opposite sign of Capricorn, does), instead of just her own.
Hope that helps.