It isn't selfish to look after yourself and you are obviously struggling with your own wellbeing just now.

It's difficult being away from home and family, especially if it's for the first time. Your sister is having a rough time with her health, yes, but that doesn't negate your problems. You cannot be expected to just shunt them aside because someone else is having a worse time than you are. Your problems are just as real.

You say you aren't thinking about her, but it's clear you are upset by her diagnosis. Maybe you don't think about her because it frightens you that she is so seriously ill? Sometimes if we don't want to face things we shut them out. Your mother is probably worried sick about her, but she shouldn't put a greater burden on you because she assumes you are OK.

Maybe you could talk to your sister? Even if it's only a phone chat to discuss both your problems - she's probably feeling alone and isolated too and might need you more than you think. Maybe you and your sister could help each other?

The other option is universities usually have an excellent counselling network. They are very hot on students who are finding it hard to adjust. They aren't just administrators - it matters to them if students are unhappy or not and they won't judge. They can also give you any practical help you might need, even if it's only someone to work things through with.

Good luck and I hope I've helped a little.