Allot of times when we grow up in a world where matters are handled less ideally than they should, we invariably inherit some of those qualities because somewhere inside ourselves we have conditioned our reasoning to view those moments as "normal", because that was the only choice we had as kids. That creates allot of disinformation to our emotional network that often makes us act out in similar manners because its instilled in us during our formative years.

In terms of you worrying about what will happen with your sister, I will say to you what I say anytime I see that in my family or elsewhere. Change will happen when one or both have had enough. Whatever the misery gauge is from your vantage point, there is another gauge in that relationship that is justifying that bond between them. Until those justifications lose their hold, nothing you can say or point out will change what both of them know intimately better than what you observe.

I feel for you tremendously, but that is human nature. We can justify anything, even when we're suffering and have the means to stop it. That emotional switch is a complex and powerful force that impacts how we see everything, even when its self inflicted. I wish you the very best and hope things work out for your sister.