It is very common in siblings of children with disabilities. I have a disability myself and it is painful to see how that affected my family, particularly my siblings. I acquired my disability when I was 17.
On the other hand, my brother has serious mental health issues and sometimes like you I think of how easier my life would be if he were dead. And sometimes I get angry because my mom, dad, and sister have already passed away and I am the only one dealing with my brother.
Therapy is helping me to manage my feelings and interact in a way that is acceptable (according with my principles and ethics).
It is not easy, but there is hope.