TLDR: My mom wants my fiancé and I to let her soon to be ex-husband / my stepfather live with us for 3 weeks while they finalize their divorce. AITAH for saying no?

My mother’s in the process of divorcing my stepfather. They’ve been separated for 4 months and made the final decision to call it quits last month.

He’s been staying out of town during their separation but is coming back to the city for 3 weeks to finalize everything. My mom and I briefly discussed having her stay with myself and my fiancé during that time so they wouldn’t have to stay in the same house.

Out of the blue two weeks ago, she asked if her soon-to-be ex could stay with us for the 3 weeks.

We said no, and my mom is very upset.

She insists he “sees me as a daughter” and I’m “the closest thing he has to a child” so I should be okay with this. She keeps bringing up that he was nice to me, wasn’t abusive, paid rent, mortgage even when I was in my early 20s and my fiancé and I stayed on and off at their house for a couple of years.

She’s now even implied the divorce settlement won’t go her way “if he thinks we are ungrateful people.”

He is refusing to stay at the house he owns with my mom because going back would be “too upsetting” for him. While I’m grateful for the financial support in my teens/early twenties, he and I were never close. He came into my life when I was 15. The transition between her marriage to my father and my stepfather was difficult and traumatic for me, and involved me getting trapped in several lies and constantly being put in uncomfortable situations, and I was expected to just go along with it. I don’t want to go back to that again.

My fiancé and I also just moved into this house and we’d like to have people over, celebrate his birthday, etc., and we wouldn’t be able to comfortably do so.

I feel like having my mother’s ex-husband stay with us for 3 weeks is just an extremely uncomfortable situation, and I don’t think his negative emotions should be my responsibility to bear.

AITAH for refusing their request? How do I even communicate this with my mom, as gently as possible?