This past weekend, FMIL said about the neighbor girl (she is in her 20's) going. I mentioned she isn't invited and she said of she will be FI's sister's date! I dropped it then, but out of our siblings only 1 is in a serious relationship, so she was the only person getting a date.
If his sister started dating someone, sure she would be allowed to bring him, but I'm not willing to have my parents buy yet another plate for some person I really don't know. Fi agrees no dates for siblings, I don't think we are being ridiculous.. are we?