Well, Alex, I think you should stand your ground and next time they're not sanitising the injection site, you just need to make a comment about that, demanding that they do. Yes, you'll definitely get the looks and the odd comment, but it's about you and your mum, so I'd simply ignore them. And if there's an issue, calling the manager in almost always works, and if it doesn't, you just need to walk out, make a complaint and come back when things are improved.
I actually still wear a mask in shops/workplace on a daily basis and the only people that ever commented was the petrol station man (who made a comment: you're still wearing a mask) and my local priest (and I'm very disappointed with him in this respect, as there are lots of vulnerable in the congregation), though he's used to me wearing a mask now and makes no more comments.
Honestly, why does it bother people if you're wearing a mask or not? I find it quite extraordinary.