It sounds like she's actually doing her best to comply - the pertussis shot and the testing different areas for mrsa - BUT she's clearly pissed off and trying to push boundaries ASAP.

Initially I thought "well, she's at least trying" but then I remembered Motherfuwch and how she seems reasonable to everyone else... And I know damn well she isn't but she hides it.

I'm on your side, if that isn't clear! But be careful as MIL is making a good attempt at "I've done everything, woe is me!"

I'd be more worried when babies come home, and she moves in to touch them without permission. I have a friend who had twins at a similar gestation (is that the right word?) and while I was welcome to visit she provided hand sanitiser before touching the babies - and I totally respect that because although I thought "they've got a cat! They've got x y and z germs all through the flat!" it was THEIR babies and THEIR choice and if babies got ill, they'd need to know they'd done everything they could.

Same with you - you set the rules that you need, and make sure they happen. Hopefully your babies will be fine, but I understand that you need to know you've done everything you can. Maybe explain that to MIL? If it doesn't resonate at all, you know you'll have to batten down the hatches and she isn't just being over keen.

I hope what I've said makes sense? I'm running on empty atm (waiting for pain meds to kick in).

Finally HUGE CONGRATS ON BABIES' ARRIVAL! May they bring you much love and joy.