@TeaK:
What exactly are you afraid they won’t understand? According to your culture, are you as the eldest daughter responsible to single-handedly help them, while your younger sister is spared of all duties?

That it’s selfish and that I only think about myself (I’ve been told that before). Their excuse for my sister is that because she doesn’t know how to do it (things they keep asking me for help with like filling out forms and stuff). She’s 20. When I was her age, I had to figure out a lot of things on my own to help them, and I don’t understand because nothing is hard about it as long as you can read English. She knows how to do it if she puts a few minutes into it. She just doesn’t care and just gives the excuse that she doesn’t know how and parents let her off.