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The original was posted on /r/AmItheAsshole by /u/TriggeredCrusader_ on 2023-09-26 18:30:25.

So a little context, I (M20) am renting from my future MIL, and I live with her, her husband, my fiancee (F20), and her youngest son (M18) who has down, and minor autism. Before we met, my fiancee would take care of him like a second mom. He spends most of his time watching tv in her room, because he can't really go outside by himself while everyone is away or working.

At the end of the workday, my fiancee likes to change into more comfy clothes, which includes taking off her bra, so when she does change she's almost naked...

Her mom is really strict, and asked/told us to while we live with her, we will not share any kind of intimacy until we're married (including watching eachother get dressed) to which we agreed.

She usually changes early, and her little brother is usually still in there watching tv, never gets kicked out. I didn't think anything of it because... well he's got autism, and I figured he wouldn't break his hyperfixation on the tv anyways.

One time we were doing the wedding registry together, and he was watching tv while we were in there. She says she wants to change but is ok with me being there, "as long as mom doyfind out we're good". I mean we're going to get married for Christ's sake. Anyways as she starts stripping, her brother's attention shifts away from the tv, and he stares at her for the whole time she's getting dressed. I expressed my annoyance to her and she replied "he doesn't know anything, he's just a kid"

Then one time i come up to go to the bathroom at 2am (I sleep on the opposite side of the house from her) and I walked in him whacking off in the bathroom. I only saw because he goes with the door open, I guess he doesn't like the claustrophobia or something.

So now I'm upset, like sure, down and mild autism will leave you mentally underdeveloped, but he's still a guy, 18 years at that, and that sexual part of us is wired into our brains. Sure he may not understand to the full extent what's going on, but staring when she changes in front of him, understanding enough to jack off, coupled with his increased hostility towards me when he realized I wasn't just a friend and I was going to take away "my Rachel" as he refers to her is Starting to trigger my jealousy.

I don't know enough about down to make a judgement call on this but I do know that the autistic people I know tend to hyperfixate, as well as lack the ability to filter. When I raised how I was upset to my MIL she got very upset at me, uncharacteristic of her and said everything was fine and on e again "he's just a kid"