Relatives that ignore rules about you know what - how to handle?

[Sittin]

PILs want us and SIL to go for lunch on Sunday. There would be 7 of us so I’ve said that I’ll do something else - they insist that this is ridiculous and I should just come. SIL is in London and says she’s going to use her ‘common sense’ and come anyway as ‘she’s allowed on a bus and to go in a stranger’s house’ so what’s the difference. I have no idea what the stranger’s house comment means. PIL are over 80, MIL has cancer. I’m a teacher, DCs at big schools, school bus etc.
I don’t know how to handle this. I’m really 😡 that they think I’m the unreasonable one.

If your SIL is in London, she will be Tier 2 by then anyway & not allowed to mix with other households indoors.

This. Quite simply.

Bluesheep - I know! And so does SIL, she just doesn’t care!

sittin yet another person who thinks the rules don't apply to them. People like this are everywhere, unfortunately.
Selfishness is more of a pandemic than covid.

You can't make them understand your view point, you can't stop them talking about you and your opinion. You can't tell sil not to go even though she is probably the higher risk to them coming from London on a bus.

Though theoretically the risk isn't lessened by you not going because your other household members are there. None of your family being there would protect them more and prevent spread and uphold your position.