DP has two siblings. His parents would like to do something to celebrate his mum's 60th at the start of May so we are looking at outside with the rule of six. One sibling is single, but the other one has a girlfriend of about a year, so nobody has seen her much in person, although we have met her over Zoom etc.
Basically, the girlfriend has not been invited to the celebration, because that would make seven. I feel very guilty that I am invited and she is not. I'm not sure there is anything to be done at this point though. I don't think me making an excuse will help at all, because she would still be a 'backup' option. I just think it would have been better if they had kept it to the five of them!