Meh, sod em. If they are too worried about colds one week but are suddenly okay the next time, just tell them no, you wouldn't want to risk them.
My MIL lives on the next street. We have barely seen her since lockdown started. She never wants to risk it. That's fine. We're the overly cautious type anyway. Washing hands, cleaning shopping (last year anyway), DH stopped car pooling even when it was allowed, we don't go out at all, and we rarely go shopping further than the village shops etc. When one of our kids was isolating from school but negative for every test, MIL wouldn't even accept something we were dropping off. We had to leave it outside in the back yard. It had come straight from the shop too, and hadn't even been in our home.
SIL however doesn't really bother. Their DS's girlfriend caught Covid last week from a night out and as far as we could tell, the only ones isolating (from their household as they all live there) were the son and his gf. SIL and BIL have been going about as normal.
They've had parties, they've been abroad, they head out shopping all the time, they love a good house party and day to day, masks are optional and hand washing is never done (and I mean they do not wash hands after going to the toilet. They are proud of it and are happy to say it. Ew)
Now all this doesn't bother me (except the hand washing or lack thereof. It's fucking disgusting!) because it's their business. If they don't care about Covid that's their personal choice and nothing to do with us, BUT, MIL will go and visit them all the time. And stay over too.
Kinda hurts DH to be honest. So now he doesn't bother. It's not even that we don't get on with MIL. We really do but DH can't wrap his head around his mum's thinking at all.