Sister (7 years older) had a kid in college, couldn't take care of it, so our mother had to adopt it.
The combination of my sister's bad decisions, inability to budget her money, constant need for financial aid from our mother, and the passed burden of a child has resulted in my mother and step-father being strained thin when it comes to their finances and marriage. They're not falling apart, but its obvious how much its affecting them. This isn't just one shitty year, this has been going on for over a decade now. A few examples:
One day, my sister calls up my mom and says she doesn't have a car. She sold her car to share transportation with a guy she's been constantly breaking up with and getting back together with for about three months. Surprise surprise, they broke up. She just got a nice job and if she doesn't have transportation she will be unable to work.
My mother decides to buy a new car and give my sister her car (2013). I suggest that she simply buy my sister a cheap used car if she's going to help her. She says she'd rather buy a reliable new car for herself and give my sister her 2013, since she knows the 2013 is in good condition and will last for a long time. I suggest that since my sister fucked up and she's going to buy a new car anyways, my sister should get my car (2003 in decent condition) and I'll take the 2013 that my mom's been driving. My mother sees reason in this, after all why should my sister be rewarded for doing something stupid, and goes to inform my sister. My sister goes off on her and badgers her until my mom just concedes to avoid dealing with it. Two months later my sister totals the 2013.
Now my sister is pregnant again. She was dating some guy for a few weeks, though she said she's known him for years, and she insisted that this was going to work out and that she wanted to keep the child. Now, two or three months later, they've broken up. Apparently he's going to stick around to help take care of the kid, but I'm not very confident that its going to turn out well.
She's not mean to me. She's actually quite nice and loving, and we can hang out together and genuinely enjoy one another's company. Yet, she's unreasonably shitty to our mother, which pisses me the fuck off since my mother has done so much to support her. Yet the moment I try to address this, my sister closes herself off and either gets really defensive or just doesn't listen.
I feel especially bad because a selfish part of me feels like my sister stole all the safety net for herself. She fucked up for so long and so much and sucked up so much of my mother's money that I don't get to fuck up now. I can't take risks, because I don't want to be and pretty much can't be a burden on my mother.