Why am I even a big sister?

When the three of us were kids, they used to look up to me. Then it got around when I turned 12 years old is when shit started to permanently change. My sister is 14 and my brother is 12. I've been mistreated by them since I was 12. I treated them with nothing but love, only to get treated like shit in the end. I have tried being the big sister. Now, I always question my actions around them and mostly asked myself, "Why do I even try?" We don't have a single happy memory together, we have none. It's been hell for years now and I'm to the point where I'm going to disown them because I can't be a big sister if I'm not allowed to. I want to officially quit being the big sister for both of them. What's the point of being a sister when you just can't? We do have a 4yo sister who I love so much. Hopefully, she'll allow me to be her sister when she gets older and understands more.