If this is actually true and went down as described, I actually wouldn't be as quick to freak out without hearing what the brother had to say about the situation. Intellectually disabled adults are still adults, and ones with less severe disabilities should be allowed to partake in adult activities if they want as long as they've had comprehensive sex ed including consent and whatnot. Hell, years back there was a documentary on Netflix about a couple with downs getting married.
It sounds like the brother is capable of speaking for himself, so I'd want to hear what he has to say about the situation (and certainly his opinion on it is the one that matters most).
Speaking of adult tards having sex, Kim from the sammieverse posted a tiktok featuring a cap of her schizo methhead baby daddy meeting his dead flipper baby via jail video chat. No, none of those are inaccurate descriptors