Blocking off time first is key, like a standing friend date every other Saturday. Sometimes you just have to put your foot down and say we already have plans. Maybe set a hard limit, like only one weekend a month for in-law stuff.
That "leave zero room for friends" part is a tough spot. You have to block off time for your own plans first, before the weekend gets claimed. It really comes down to you and your partner getting on the same page about balancing family time.