Don't give into that s**t. Your parents are people and if they wont treat you right then they can go straight to hell. Especially because they're your parents. If they wont treat their own family like people, they don't deserve a family.
You need to remember that just like them you're a human and deserve the same respect. If they're willing to disown their own child for something silly and miniscule as just like wanting to be gay or something, perhaps they aren't even people, and don't deserve your respect.
You shouldn't feel guilty for living in a complicated situation. My parents have been divorced since I was three, and the one person my mother dated who I thought I could trust got shot by the guy who used to live upstairs to us, who harassed us for years. I felt sucky and horrible for all my life and I never wanted to express how I felt about all of it because I didn't want to annoy other people with me talking about my bad situation. And all it lead to was untreated depression and other mental issues I don't feel like talking about.
NEVER feel greedy for wanting to help yourself out, feel greedy for taking from others, I can confidently say there's a HUGE difference. If your sister wants to help you, let her.