Your story is one that is shared by many siblings of chronically ill or disabled children. It does make sense that most of the parents’ care goes to the child who needs it so much. But that is small comfort to the child who feels left out or second in parental affection. Your experience of emotional neglect was also increased since your mother’s job was taking care of other people’s children. She was probably doing the best she could. But that doesn’t mean that it was enough for you.
Your challenge at this age is to both have compassion for your parents’ struggle and compassion for the little girl who was you. I think you need the support and guidance of a counselor to help you with this. After containing your feelings for so many years, you may be worried that you will be inappropriate or that you will explode if you let them out. A counselor will help you explore your emotional self and reclaim the normal range of feelings for someone your age.
I wish you well.