Covid breach

My parents are due to celebrate a big anniversary. They are both in their 80's My sister has planned to go away to a country on the UK's Covid warning list which means she should by law quarantine for 14 days on her return. However she is planning to come to my parents party within a week of her return. She says they will get a private test and then come along. I have issues with this. One they'll be breaking the law, but also they will be putting at risk the health of the rest of the family. We can only be a small group anyway but I think this is crazy. She is adamant that she will do this and I don't know how to deal with it. My parents view is that if they get it, then so what. But surely that's not the point. What can I do to avoid a complete family fall out at a time of celebration?

There's not much you can do about your sister's stupidity other than postpone your event by another 7 days so that she can quarantine properly. People in their 80s are high risk and anyone who puts them at further risk is seriously remiss, and never mind it being illegal. The Covid has forced many changes and people still get on with their lives but safely. Given these circumstances, being a week late or a few days out shouldn't take anything away from your parents big day.