I noticed an absence of threads about games that contain scenes / routes with this fetish, so I'll post the ones I know. They're free because piracy, but consider supporting the devs if you enjoy them.
Games by Kirsess, all have at least mom. Short and not terribly written, but the problem is a good NTR story takes some time to introduce and develop characters, whereas these just rush headlong into the sex.
https://f95zone.to/threads/mom-is-a-meteorologist-parts-1-4-kirsess.44148/A Wife and Mother. This one's the opposite, some call it the ultimate blue-balls game because after years of development the protag still hasn't really had sex with anyone besides her husband. The bully subplot is based on blackmail and there's not much there yet, but it's definitely a good start that's worth checking out. I believe the next update is going to advance the Aiden subplot.
https://f95zone.to/threads/a-wife-and-mother-v0-115-lust-passion.5944/The Tyrant. One of the better games for this fetish, though the "bully" Davide is more of a criminal lowlife. You can not only have your mom NTRed by him, but also one of your sisters and a potential love interest.
https://f95zone.to/threads/the-tyrant-v0-9-1-saddoggames.6025/Henteria Chronicles 2. This is not exactly
bully-mom story, but it could be even better for some and it does feature a bully. I honestly don't want to spoil it, because it's a poignant story about the protag's loved ones being corrupted by bad people, that should be experienced properly. If you're going to play it, my advice is set aside at least a few hours, don't go into it horny, play the long prologue and don't rush through the dialogue. It's prime NTR that relies on its writing rather than the art. The sex is all pixel-art with RPGM sprites anyway, so it has little value as a quick fap/schlick.
https://f95zone.to/threads/henteria-chronicles-ch-2-the-law-of-the-tribe-update-6-fix-2-n_taii.61811/There is a bit more, but honestly you'd be scraping the barrel. The rest are either too lame or have very, very little content of this fetish. If you broaden the definition of "bully" to "douchebag that wants to screw your mother", there is some more. Usually the "commit incest or get cucked" variety like The Tyrant, but worse.