I have an older brother with profound disabilities, and a younger brother also. My parents were the first generation to keep their kids at home rather than send them into institutions, 1960s. They were brave people.
We had a wonderful childhood, although I know now it was very hard sometimes for my parents. My mum always told me and my younger brother that we had not been born to care for our older brother, that she wanted us to just love him, advocate for him and look over him as we would with any other sibling. We were never expected to do any hands-on care or even push his wheelchair. I know now that this was a conscious decision by our parents. Despite this, I can distinctly remember, about 7, the realisation of what death was, and what it would mean for me and my brothers.
This was all fine, apart from the fact that younger brother never really had any interest in anyone except himself, and grew up to be a very difficult person with no empathy and a very difficult relationship with everyone in his family and everyone he has ever met!
My mum became very ill for a while when I was about 18, and I stepped up and helped with whatever was necessary, from housework to helping toileting my brother, because I loved my brother and my mum and wanted to do it. Younger brother never bothered.
I can honestly say that every single important decision I have ever made in my life has been dictated by my sibling to some extent. I broke off an engagement as the man did not like my sibling. I never went on the gap year trip with my mates as my mum was sick. I married my husband (partly!) because him and my brother were instant friends and are still very close.
Things turned very bad when my mum died suddenly at 65. My Dad could not cope with brothers physical care at all, and a lot was left to me. Younger brother has never really done anything practical to help, and yet he resents the relationship we have.
Things are great now after a very very difficult time, our brother lives in his own supported living flat with lovely carers and has a really nice life. I take care of his finances and watch over things, but we have a brother and sister relationship now instead of a parental/child one like we had to have for a while. Younger brother visits now and again for the odd half and hour, but has no meaningful place in his life. If I had not had my wonderful husband and my cousin to support me, I would have coped at all.
I have gone on too long, but I suppose my point is, don't have a child to support the other sibling, because the relationship my mother wanted for us never happened, and may not happen with yours either. Have a child because you want one.