I have a very autistic daughter (who is technically intellectually disabled but really more like an alien intelligence), and while I have no desire to have sex with her I don't see why that couldn't be included in an incest game given that there was consent.
Most people discount the disabled when it comes to sexuality as if they are forever toddlers, but they are human and as such have normal sexual desires. It's unfortunate for them because they end up being denied that basic human need and shamed for expressing sexual desire. On the other side of the coin, any person of average or above IQ who would admit to being willing to have sex with a disabled person is treated as a perverted monster.
I considered a character using a wheelchair, but it didn't make sense for the level of technology. I don't plan to produce any incest content, but Quula and Quola ride the line as they are clones raised together (they will also display autistic traits though June's will be much more pronounced).