FI has an uncle, late 40s, who is mentally disabled and lives with FFIL. He isn't likely to come, but my instinct is that he is an adult member of that household and should get his own invitation.
Problem is FI doesn't really know him and doesn't talk to FFIL unless he absolutely has to, so I'm not exactly sure what his specific condition is. It sounds like Down Syndrome or something similar - basically a functioning adult, but not able to hold a job or live on his own. I don't even know if he can read, or if that even matters. Am I being patronizing assuming that he might enjoy getting mail addressed to him, filling out his RSVP himself or with help, etc? The whole point is I don't want to treat him like a child, but the previous sentence is exactly my logic for sending a separate invitation to my 12 year old sister. Also maybe it's just a hassle for everyone involved and I look like a jerk.
I know I'm overthinking this (my #1 hobby!) but humor me: based on the info I have, what would you do?