Technically not a mom story. It's about a bullied teen and I guess she would be his sister-in-law. Her husband, his brother, commit suicide and they have fallen into a depression. In addition to that, he is being bullied by kids at school (kids surprisingly younger than him). She goes over to talk to them and gets won over and pulled out of her funk and into, well, a different kind of funk. It contains some weirdly young stuff, most of the site is Japanese (where that's common in fiction) though they're adding a ton of English content.
It's a surprisingly good story. There's definitely things I would change (If we were swapping her out for a mother, would it be better if the father died or a sibling?).
Anyways here it is, on Pixiv:
https://www.pixiv.net/novel/show.php?id=12172813After finding that gem I looked around and found a couple others more close to our interest. They aren't as good but the writers were trying.
Mother Married to Bully part 1 though it looks like they've given up on any part 2:
https://www.pixiv.net/novel/show.php?id=9915798And Mommy's Humiliation, which seemed like it was going another route by having the bully be not only a cousin, but a girl.
https://www.pixiv.net/novel/show.php?id=11343219https://www.pixiv.net/novel/show.php?id=11352782Spoiler and Warning: It doesn't get to sex. It's possible this is going to go an entirely nonsexual way but so far it's up in the air. The artist has some (3d) images in his gallery that go along with scenes in the stories.