Guest List Tension - Mother of Bride fiance's family

Hi all!
I'm hoping for some neutral perspective on a hot topic of discussion between my mother and I... To give a little background, my mother is recently engaged, set to be married next year (3rd marriage). She, my father and my ex-stepdad split the bill for the venue which includes food and drinks. My fiance and I are paying for the flowers, photographer, decor and DJ. My mother feels very passionately about inviting her future hubby's family (siblings and his children plus spouses) to the tune of 20+ attendees. My FH and I have only met 3 of the afore mentioned relatives. She insists they will pay for his family members. Here's the issue - my FH and I do not really feel comfortable sharing such an intimate occasion with people who we do not know. I agreed to invite his parents but a line needs to be drawn somewhere and I'm having trouble finding a middle ground. I just learned today she alreadry invited them all before my FH and I agreed to inviting them. She is now VERY upset with me because I will not agree to inviting people to whom I am not related and have never met. She has gone as far as calling me an embarrassment among other not so nice things... I don't know what to do. Please help!