That rule sounds great OP and I hope your DH can grow a backbone and present a united front with you :)
This has reminded me that my Dbro and I used to get sent to our GPs for a week when we were kids, at my GM's insistence. We hated it and after the second time, plucked up the courage to say to our mum that we didn't want to go back there without them (courage was needed because my dad's mother could be very manipulative and controlling, and hated my mother, and as kids we were very aware of the undercurrents and tensions swirling around the house).
Rather uncharacteristically, my mother took what we said on board and we never had to go back there again. It was one of the few times she really had our backs and I was so, so grateful and relieved. A few years ago (so thirty years after all this happened) my grandmother mentioned bitterly how much my brother and I "used to love" coming to stay with her but then my mother "put a stop to it".
Initially I felt guilty (which is my grandmother's special superpower) because I thought "If only my brother and I had just put up with it and not complained. Clearly it really upset my GM or she wouldn't be bringing it up. And my poor mum, having to be hated all these years later just because we complained."
But then I remembered that a) my GM wasn't going to like my DM whatever she did. Us coming to stay a few times is a drop in a bucket for all the other stuff, b) my GM has a memory for imagined slights and power games that would impress Derren Brown, and c) it made us feel that we could confide in our mum and she would help us if she could, which was pretty priceless. I think my mum would say the same. Actually, she'd say "I am so past caring what your GM thinks of me" 
Sorry, bit of an epic post....