Family politics over guest list is stressing me out

I am the bride, my family and friend list is naturally smaller than my groom’s friend and family list, as he has a lot of family and friends. More siblings, more close cousins and larger friend group(s) than me.

Initially, I felt sad about my small guest list but I genuinely love the people on there so no harm, no foul. Got over it after some time.

However, my mum seems to have a massive issue with the list and thinks it should be 50:50 split. I’ve tried to explain that my fiancé has a different life situation than I do, I’ve already accepted her friends to the list (she has about 15). Brother chipped in and also agrees with my mum. I was starting to see their POV but my fiancé is still adamant we shouldn’t artificially expand the guest list, which was where I also stood initially. She’s even threatened to not come twice now because of this.

What do I do? We’re getting closer to when we need to send save the dates and I just want to move on from this?

TLDR: I have a smaller guest list than my fiance, ultimately I don’t care anymore but my mum does and is causing issues with it. What do I do?