Well... Your sister IS getting divorced for selfish reasons. Let's look at the definition of selfish:

lacking consideration for others; concerned chiefly with one's own personal profit or pleasure. Going along that, she is being selfish. Although her kids and family are sad, she's ending the marriage for her own sake (marriage is "dead", and is looking for love, according to the parents). I cannot see any ambiguity , so I agree with you.

The real thing is here, is that you don't believe she should be selfish, that she should be considerate of others and keep being married. I think there are a few problems here:

• She has been married for 10 years. Don't you think she knows better than you? All you know is their outside appearence, because you have never been "inside". After seeing many couples that shouldn't be, be, and many couples that should be, don't, I've noticed that no matter how much you're around them and how much advice you give, sometimes things just don't go how YOU think they should. I believe that's because there are things you just cannot "know" in a couple that doesn't include you. Have you ever tried explaining one of your problems to a friend, they suggest something, and you just think "It's more complicated"? That's every single problem in a couple, because a problem isn't just a problem, it includes all the baggage, history, and scars between the people.
• You say you've never experienced divorce. I feel like, as with anything, you are scared of what you don't know. This is fine; as someone who, although has never had one, has had their parents and who SO's parents are divorced, I'm here to say that divorce IS the better option than staying together sometimes. This is anecdotal, but I personally wished many times my parents would get divorced before they did, and although it was tough at first, like all changes in life, it is way better now. Maybe if something keeps breaking, it's time to change it. Is it better for her kids to have a dysfunctinal family, or two seperate ones?
• Most importantly, I think that, if anything, she deserves to be selfish. I can't think of any other situation than a couple where someone not just can, but SHOULD be selfish. Sometimes, just going by "the list of goods and bad" just doesn't work. I'm not saying be a dick to your SO; I'm saying, if someone with "perfect qualities" doesn't fancy you, but an average/normal person does, go for them. A couple is the most personal thing you have, you might aswell listen to yourself.