Yes...depending on the disability, it can greatly impact custody.
Obviously, someone who requires assistance themselves, would be unable to properly care for a child and therefore should not have custody. It doesn't mean that they are a "bad" parent...it's a question of safety.
Also, someone who has a degenerate disability...meaning that they will get worse, should also not have full-time care of a child...again, it's a safety issue, NOT that they are a "bad" parent.
Maybe if related the nature of your sibling's diability, I could answer your question better.
If you air your dirty linen in public, expect people to comment on the skid marks!