Feeling ever so slightly miffed

Sounds like she might liven things up a bit! I have to say that you do come across a bit Hyacinth Bucket. Is there an Onslow in your family who is kept under wraps?

H1954. Does it really matter?. Be nice to your guest who might just feel a bit overwhelmed by the occasion. You might even find that you like her/him. She/he might be a lot of fun as they will be more relaxed than those who worry too much about how one holds their knife and fork. Enjoy your party and relax, sounds a teeny bit snobby to me.

Oops H1954. Just read your post. Difficult one?.

Of course, it was rude of them to invite somebody else without even asking. You don't want to be rude in return by making a fuss about it, though. Far better just to ignore and accept it.

H1954 You're wrong - I have got good manners, thanks. You sound a bit ridiculous.

In DHs family, partners (including new/recent ones) are always included as an unwritten rule.

In my side only named invitees are welcome.

Neither side is "wrong" IMO just different. These are the things thar come up when you blend families.

It would have been nice to ask you if it was OK H1954 rather than just assume. If they had asked you first, you'd probably have been too polite to say no but I understand why you're feeling slightly miffed.