Being that I live in Laurel, I don't really think the drive to Rockville is that far. Now for my friends in Upper Marlboro, they would have to drive twice as long as I do to get to Rockville.
I would recommend that you make this compromise. Tell your husband, MIL and family that you will have the one party only in Rockville. Say that next year, you'll have the party somewhere closer to his family, say a park in Greenbelt or somewhere. Then you have one party each year and you alternate between Montgomery and PG County so that every other year, the party will be more convenient to one side of the family. Either you can host at MIL or SIL homes or rent a space out in PG. If you need a free space, there are a number of parks that have free pavilions, first come, first served, or ones that can be rented for a minimal amount in PG. To get residents rate and not pay the out-of-county rates, you can have your MIL or SIL help you make the reservation to get the resident rate.