To be honest. I got it from 4chan. For me, it's an extension of "yo mama's a hoe" fetish. I come from a conservative and sex negative country, and mother insults aren't taken a bit lightly. So, what happens if you found out that your mom just had sex with someone other than your dad? You'd be devastated because now you know that she's a whore.
Another extension is moms doing porn, where not only she has sex for money, but the whole world knows about it. For me I like the idea where she does porn recently, when she had already established a family, not the typical "did it when she was young and needed money".
Now for this fantasy. Your mother is a whore, and she's doing it with the kid that is making your life a hell, and this with serve more wood to fire the humiliation even more. Personally, I like the fantasy of mom doing it just because she's horny and letting you get beaten and humiliated just because she likes his dick.