My mom is dating someone younger than her!

She’s 40 & he’s 29 she has 4 kids youngest is 5 years old & he claims to have 1 kid but has never seen her before because the mom is bitter. He wants family + more kids and my mom mentally can’t have anymore. She can barely handle her 5 year old she doesn’t have the patience for kids anymore I told her that she should leave because he’s a lot younger and cause he wants more kids but she said she’s willing to give him a baby as long as he takes care of her & her kids I’m pretty upset with her because they’re 2 months in the relationship and they’re already talking about moving in I’m 24 years old so this is very disturbing!! Am I overreacting??