Scared of having a child with significant needs or disability

I have a brother with significant disability (severe autism, learning disability, non verbal etc). I love him very much as does my family but he is very difficult to be with for prolonged periods. He is a 300lb grown man aged 30, and during his meltdowns (6 or so on a good day) he can throw his caregivers across the room. He has broken countless televisions, laptops, ornaments, walls etc on a weekly basis. He stims loudly and constantly and paces over and over through the night and into the morning. It is typical to be woken by him stimming and pacing at 4am. He has to watch the same hey dugee episode over and over and over at max volume - the household television can now no longer be used or switched off. Etc etc.

I have seen what having a disabled kid did to my mother - before he was born she was somewhat loving. After she became emotionally drained, cold and distant and emotionally unstable. She checked out of raising us siblings because her focus was rightly 100% on our brother. My father and her separated and the stress from my brother was a big factor. Her finances are mostly diverted to his care - she has no retirement pot etc & lives very frugally. the list goes on - all in all she has had a really difficult life and while he is loved by us all there is no doubt he has had a profound effect on her mental heath.

I am terrified of having a kid with similar disability. I already have 3 healthy kids and i have also been genetically tested to confirm i am not a carrier for my brothers condition (the autism is a feature of it). However i am approaching 40 and want to have one final kid - but i am terrified of my advanced age causing something similar. We are not in a place financially, emotionally or physically where we could cope.

How do i get over this fear?