Reception - Reserved Tables/Seating Charts

We have only a couples table with open seating for guests. We have Our set up is for 8 guests to a table, however, each table has room for 2 more guests. His mom requested I reserve tables for our wedding party and family. I considered this but it isn't possible without breaking up our wedding party and making (my) family uncomfortable. She doesn't seem to care and still insists I reserve tables for family because his sister didn’t at her wedding and there were issues. Also, her definition of family is not just parents, grandparents & siblings - but extended family of aunts, uncles, cousins & close friends. This requires reserving 50% of my tables. If I further included our wedding party, that reserves just under 75% of my tables. I strongly feel that the rest of my guests, who are just as important, will be isolated which is rude. I pointed out that there's no "win" and my plan holds guests accountable for their choice of seating rather than me. How else can I politely tell her no?