There was a similar situation to this in r/jnmil, where the parents of a couple got together to purposefully break up the kids relationship. I recommend reading that thread for advice.

Quite frankly, they know what they’re getting into. It’s not wrong for the mom to have feelings for someone, but considering you two have been together longer, she’s being selfish and inconsiderate. She needs to carefully consider the situation and see how it affects you, but she has and doesn’t care, or she’s fighting against that idea for this chance at happiness.

Either way, I think your mom should go to counseling before getting into a relationship with her daughter’s future FIL. She’s been in a string of abusive relationships, and lost someone close to her, so this could potentially be a way of her coping. Which is incredibly unhealthy.

You and your boyfriend have been together longer. This clearly isn’t gonna change, so I would argue your mom is being selfish and putting herself over her daughter.

I would advise going VLC before cutting them off to see if this relationship between y’alls parents actually goes anywhere.

Also, the likelihood of them having a kid is pretty low. Based on the ages, your mom is probably gonna start going through menopause soon. If she hasn’t already and gets pregnant, the kid is likely going to have some disorders. That’s a huge risk and too much energy for 2 adults at that age. The kid would graduate high school well past their retiring age. I wouldn’t worry about that.