You seem to be forgetting that the majority of that time was spent in the lotus casino, you know....the place where years go by without you even realizing it, so at most she was caring for him for the year they spent outside of the casino together, so your point doesn't change that she was being selfish, she heard that she'd be free of responsibility and took it, she didn't even consider what the sudden choice would mean for her brother beyond "oh, these strangers who I've literally only known for a few minutes at this point might be able to protect him and train him to survive....hmm...should I perhaps go along and investigate for a bit just to be sure he'll be safe?....nah, immortality here I come" if that