I doubt I would have turned anyone away - as ticked off as I would be at the mother in law I would not want to take it out on the guests.
But she mentioned she was a rule follower who is greatly concerned about Covid. If that's true then telling mother in law immediately that she couldn't come with that many people or if that didn't work turning them away is what she should have done. I'm just not seeing how hiding away when they came in was a great response. It may be effective but not the way she wants it to be - I know if I received that email it would be a loooong time before I would be going over there again.
Sometimes people who have a legitimate gripe or issue derail themselves in their response. Maybe it's just my personal impression but she comes off as kind of tedious, the kind you have to "be careful" around. The follow up email didn't help.
To be fair I would blame the MIL for that and I wouldn't be accepting any of her invitations on behalf of someone else either
She definitely has mother in law issues - no doubt.