What should i do?

So weve sent our invitations out and are slowly getting rsvps back from everyone. Last night we ran into an issue we weren't expecting. Context, we only gave unnamed plus ones to the guests who we knew were talking to someone at the point of planning, which we verified with those folks then. Now one of those was my fiances cousin and he doesnt have anyone now which isnt a big deal. But one that we didnt give a plus one to is my fiances sister, well she never said anything about it and apparently she was upset at the fact that she didnt get a plus one. So instead of talking to us and asking for a plus one she just went and asked her cousin to bring her best friend as his plus one. Now we have a situation where my fiance and I are upset with his sister for this, and his parents are upset with us that were mad at her.


We are so lost in what to do, we would like to tell her no that her friend isnt allowed but were worried about starting more issues with less than 50 days to the wedding