I'll start by shouting ARGHHHHHH!
My DM and Step Dad live an hour away. Like most people, my DM has felt a bit down over lock down feeling she hasn't anything to look forward to, it is also her 70th soon. I suggested myself, DP and our DC's 20 and 15 go to hers to celebrate. So, that's 6 total, all good.
DM loves Karaoke, 60's music, and cocktails. We had planned I would treat us all to a Chinese Take-away (another of DM's favourites) and I would make a massive chocolate cake. Eldest coming home from Uni especially to celebrate also. DM has been very excited, sorting her playlists and I also believe a naff disco ball has been dusted 
Anyway, DM just phoned saying my Step Brother and his wife have just been over for dinner, and are also coming over for her 70th.
This is 8 people and not allowed. In addition, my Step Brother is vegan, and his wife a celiac, neither will eat the planned take-away, as such, DM just told me she felt put on the spot and has just asked me to make food suitable for them, suggesting I make variations of lasagne that we can all eat.(so a meat version, a vegan version-and one suitable for celiacs I assume?)
I put the phone down a bit like a rabbit in the headlights-the initial plan has gone out the window, and I absolutely don't want to break the covid rules.
Do I call her back and say our family of 4 can no longer come? (she'll be upset) Do I call my Step brother and ask if he can visit with his wife to celebrate another day as we had plans (I would feel REALLY horrible doing this, as he is a lovely bloke, and I don't want to hurt his feelings)
But. It's 8 people! I also don't remember agreeing to cater for 8! So the initial plan has changed anyway. What can I say to cause the least hurt/annoyance?
Also bloody gutted.