To dislike his female friend?

He`s chosen you not her, remember that, though they have that friendship history together.
I agree, be above all this, very difficult I`m sure, don`t let her too close, but be polite and pleasant in a distant sort of way.

I had this happen to me years ago, a friend of my DP shared private jokes with him when we were all together and kept ringing him up. When we moved in together she got the message. I must say I could have punched her in the nose sometimes! Kept my cool though. She moved away and vanished off the scene.

Oh you’re a big person, I d find it hard to not feel jealous when they are off on their jollies, although you are obviously very trusting of your partner and he has not given you any reason to doubt him I think she is trouble and has obviously seen a future for herself with your partner hence the reason for wriggling in through his family, her behaviour is disrespectful of you, she definitely has a thing for your partner
Does he see her behaviour as ‘suspect’ or is he just a normal unobservant man
I don’t like her on your behalf ?
I d make sure I sat away from her at your meals and as apricity says no providing her with fuel by confiding anything to her
I hope you move in together soon and be aware she may up her game.... how very unpleasant for you but don’t let her upset a good relationship

Sounds very much as though this other person is rather peeved by the positive development of your relationship with DP. He appears to have made his feelings about you quite clear (in public as well as privately) and that he sees a closer future with you. This other person may have had some hope it might be her hence trying to ingratiate herself with his family and suggest a prior or larger claim on DPs life.

Stay pleasant, polite and a little distant and don't tell the other person anything personal that you wouldn't tell the whole group. Stick with the old 'if you can't say anything nice don't say anything' principle and don't get get caught up in any of the "it's just between us girls" type of conversations with this person. Anything personal you share with her will come back to bite you.

Best of luck. It's a tricky situation.

Keep your friends close and your enemies even closer! Be her friend and keep your eye on her...

I second what @Grammaretto said, trouble trouble trouble!
Take the reassurance from your DP and continue to smile sweetly when in the company of this woman - it sounds like she is upping her games as she's desperate for a reaction from you (most likely so she can do a "poor me, I'm so upset routine for attention from your partner) and isn't getting it. She sounds extremely jealous of you, but that's her problem not yours. Keep discussions about her silly games light-hearted between yourself and DP and don't let her come between you, nor let her silly shenanigans put you off socialising at these meals etc xx

I would find it disturbing and upsetting. I can't think what else you can do than carry on as normal and act as if you couldn't care less. Difficult though.

I agree with Grammaretto.

She sounds like bad news to me. A flirt and a troublemaker. The sooner you move in with DP and make it clear you and he are an item without room for 3, the better.
Surely she doesn't take precedence just because she has known him longer.
Some people are only interested in other people's partners.

It’s interesting that someone else stirred the pot by telling you she was horrified and concerned he wouldn’t be allowed on future trips. I don’t think you are being unreasonable but I’d carry on as you have been doing by being polite but keep her at arms length no more little private chats as it causes awkwardness.

No it wasn't. Will read tomorrow! ?

Sorry that wasn’t brief at all