Why is my mother so attached to my ex gf's kid that she got from cheating on me?

Get one of those family members who are on your side to lure your mother out alone to a crowded restaurant and then join them. Tell your mom point blank that she's acting selfish, choosing a stranger over her own flesh and blood. Point out that you're still young, and will have children of your own someday. Ask her if she honestly doesn't want to be involved in those children's lives? Then give her a point-blank ultimatum. Either it's you or your ex. Leave her no other options. As a parent that should be an easy decision for her.