People keeps telling you that maybe your parents treated you better than her(abused in some way) and that's why she is such a shitty person, but let me tell you about my brother.

My brother was by all standards, the favorite of our parents (they were never unfair to me or malicious, they just had more hopes for him since he was the "boy").

When he was about to start high-school, he threw the worst tantrum of his life, because he wanted to join a private school with his friends, but my father enrolled him into a national one. My brother became different from that point on, he was way more aggressive with our dad and his usual laziness became worst. One day he left all the dirty plates in the sink(we had this deal that if I made the food, he will clean the plates), my father started yelling at him for being overall lazy on everything and my brother started yelling back at him (my brother is way taller than my dad at this point, and really intimidating), my dad got mad, and kicked him out.

After that, he went to live with our mom (step mom actually but she raised us our whole life, and she had the biggest soft-spot for my brother). My mom also got tired of his laziness and sent him to our biological mother, our biological mother also got tired of him and he moved in with our half brother. Every person who "saves" him gets a version of how the previous one was the worst and he is just a victim.

He started as a great brother and son, he was definitely the favorite, we were raised the same way (he had way more freedom than me though). He became what he is today on his own. Sorry for the long post.

My point is, think really hard about how you guys behaved as children, how you both were treated by your parents. I can see anyone saying that it was unfair that their siblings got rewarded all the time (but people don't mention that it's because their bothers/sisters were "A" students while they were barely in class at all). Or "my parents always give money to my brother and not me" (after they caught me stealing from their wallets)

Imagine an scenario where you had to take care of her, that she herself is the type of person she is but living only with you. Now, how long would she be unemployed? Would she help around the house while you work? Could you leave jewelry in your room without locking it up?

If you think you can take care of her without regretting it later, then you can move in together when she turns 18.