There are no such official information available, which states that in case, of siblings if there is one child disabled then the other child will be considered to be a single child. SO you can't select the One child category because you have the sibling & as your sibling is disabled, it doesn't mean that you are the single child.

For more information regarding JEE MAINS you can consider the link below:

https://engineering.careers360.com/exams/jee-main

I hope this helps. Wish you a great eve!