Feeling ever so slightly miffed

If it’s a getting to know you event, you’re getting to know someone else and may learn more about family dynamics. You might like them too, so win-win in my view.

Just say you don't have room at the table for more people and you'd love to meet this other person on another occasion

Is this a formal sit down dinner party or informal buffet type? What relationship does the +1 have? Who told you she was immature and unsociable? Is she likely to be in your lives at other family functions?

As you say it’s a getting to know you function I think you would have to decide whether it would greatly offend and if so let it go. No point in causing ruffles simply because you are miffed they didn’t ask first.

You might like to wait and see for yourself if this person has social graces (dear lord, who says that?). They might be an absolute delight. Heavens, it's a party - what's one more guest? Hope it goes well.

why be so miffed about it ?

Unless its going to cause huge issues cos you have an extra guest, i would just go along with it, unless of course you are organising a select candlelit supper

You could show your own social graces by just going with it and making them welcome

I'm not sure what you mean by 'social graces'. Is it possible that she has learning difficulties or something like that which could make a social occasion more challenging.

I don’t think you should be shot down in flames if you are feeling put out then say you would like to invite so n so on a different occasion, but for me I like to make up my own mind about ppl, I don’t really go by hearsay, nine times out of ten I find them fun and a laugh, All sorts make the world go round

The party is so both families can get to know each other. It doesn't sound to me like a very good start if you don't want someone there. At least the invited guest won't be turning up on the doorstep with someone you didn't know was coming. Be careful, you may alienate yourself from the rest of your partner's family.

I hope you have a great time, nevertheless.

Id be miffed too. But then there is form on DHs side - we hosted a restaurant meal and turned up to find 3 extra people invited by mil without a word being said. And when I say hosted I do mean that its a meal where we pay for everyone...

Life's too short to be 'miffed' . All you can do is shrug your shoulders and make the best of it.

Just be a gracious hostess.

Who knows, after a few glasses of cheer, others may lose their social graces too ?

I don't think you're being "snobby" by using the phrase "social graces" as it covers a wide variety of behaviours. From grandpa taking his teeth out & popping them on the cloth to someone using serving platters as their personal ashtray to the person who rifles through your cupboards to pop that expensive bottle of unique booze you'd purposely hid into their large handbag - all of which I've witnessed. Wasn't me honest! As others have said be the consumate hostess, rise above it. Greet her warmly, I bet she can tell you a few secrets about yourother guests they might prefer kept hidden - could be illuminating if they behave badly or petulantly then quietly tell them so & let them know you'd been told they'd do this by their own family. Give them a chance to prove everyone wrong. We have one in our extended family-a real me me me diva and worse with alcohol! Let us know how it pans out, I think that because you're newish to the family you want itto go well, its a lovely gesture. But remember to assign any bloopers & idiot guests to 'life's rich tapestry' I hope no one is setting you up with an unpredictable guest on purpose so just sail through and

H1954 - Have you thought that maybe your OH's relative is also wanting to use this occasion to introduce his +1 to the rest of the family? From what you say this person comes with some reputation but you don't know how that reputation has been achieved, and it might be that this person wants to introduce her to a wider circle and sees your event as the place to make the point that she is important to him.
Actually, if it's a party rather than a dinner party I'm not sure that one extra body will make a difference, and there's always the chance that this relative won't turn up if he has to come without his +1 so you lose the opportunity to meet him in a relaxed gathering - and he may be not be confident in such a gathering without her anyway - he may not actually get on with the relatives who have been invited and need someone else to talk to. I would give both him and her the benefit of the doubt.

I would take it as a positive sign that this family would like to bring their +1. I would say nothing and just make them welcome.

'this family member'

You never met this extra person but you’ve allowed someone else’s valued judgement to cloud your opinion. (Shakes head)

'wanted to invite close family to a small party so both sides can get to know each other'.

Well, this is exactly what's happening. You are getting to know that this member from your OH side does this kind of thing. (S/he may not like going out on her/his own?)

Don't worry about it, and don't let it spoil your nice new relationship. I hope all goes well, and that the uninvited guest turns out to be fun.

That's super annoying. Perhaps this third party is just lonely and needs company. Use this as an opportunity to make fun of this person behind closed doors. It can be entertaining, hopefully. And next time you'll know how to extend invites to his side. Best of luck!

Perhaps this family member is nervous of meeting you or perhaps nervous in social situations and needs his partner there for reassurance. Sounds like she could be fun. Enjoy.

Salsaqueen, it’s not snobby to expect someone. Uninvited to your own home, to behave in a certain manner. If one cannot conduct themselves in an appropriate way, then they would be swiftly asked to leave. It’s more concerning that you feel social graces are not to be adhered too than believing them to be snobby. Not a snob, just not a goat at the table!

Grandmablue Thanks for your input - but my answer was directed at the OP, so your opinion isn't relevant to me

It could be seen as a bit rude but if it were me I wouldn't think it worth worth getting het up about - I don't suppose an extra person is going to make much difference.

SalsaQueen, I am certainly NOT a snob! Like many others on GN I was brought up to have manners and a sense of decorum, evidentially both sadly lacking in you!

I was miffed because I was TOLD the third party was attending rather than being asked if it was alright for them to come along. The "lack of social graces" was an opinion formed by myself and several others when we were recently in the company of the "plus one" who was loud, obnoxious, boisterous, in-drink and generally unpleasant! To that end, she remains a stranger to me as that is not the sort of person I would want to associate with under any circumstances.

The extra guest, providing she is sober on arrival, will be welcomed along with everyone else. Thank you GNetters for your comments and observations but the subject is now closed.