OP, your DD has 10-11 friends, so you must be doing lots of things right!!
You know this situation best, so trust your instincts.

OP - I would have the birthday party on Tuesday as planned and hopefully have a series of activities which can keep the girls occupied and engaged. I would have the cousin's Mom attend the party and alert her to what is up if she doesn't know already as maybe she can be sure that cousin does not pair off with gf. I would also be sure to have a structured party of activities for this group so that there is not pairing off into twos and threes. It is too long ago to say what would be appropriate but some sort of ideas"

- Free play outside
- Birthday party game outside if weather is good
- Craft time making something 8 year olds would like. - maybe decorating the cover of a notebook one could draw in or write in? Decorating a head band, making a colorful beaded neckace or bracement.
- Time to eat
- I wonder if there is some aspect of the party kids might do with food such as decorating a cup cake or even a big cookie to just take home.
- End with maybe a short movie or if the kids like to dance provide that.

You need to have enough hands on deck to keep things moving along.

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Your child is not going to make progress in terms of social skills, social anxiety if she keeps trying to get this social situation the way she wants it to be. It's not going to happen.

I would not have the cousin attend the birthday party. Have her mom plan something special and then claim she forgot about it so now she can't attend the party..

After the party is over, go cold turkey and stop the three way socializing with your DD, cousin, and other child. It's not ever going to go well and your DD will only end up frustrated and hurt. Don't explain it to DD. Just tell her it won't work it to see them. Start encouraging new friendships. Expect this to take a while and to be difficult. But this is very likely the only way your DD is going to make any growth. She is stuck in the same patterns with the same people.

You seem twisted and cruel. If kids have anxiety about friends and relationships, you don't sever those relationships. You find a way to muddle through. Cutting them off from their friends seems like it would compound the anxiety.

What is twisted and cruel is to keep insisting that her DD stick with this 3 way friendship that causes her DD stress and anxiety and constantly makes her feel like her cousin is better than her and she is in competition with her.

I agree that severing this three way friendship is what I would do (in theory). Give that the girls are cousins and if I'm reading it correctly, attend school together, I don't know how it would work in practice. I would do it for the reasons pp stated- the dynamic is damaging and OP's DD is not able to handle it a present.

Anonymous wrote:

Anonymous wrote:Your child is not going to make progress in terms of social skills, social anxiety if she keeps trying to get this social situation the way she wants it to be. It's not going to happen.

I would not have the cousin attend the birthday party. Have her mom plan something special and then claim she forgot about it so now she can't attend the party..

After the party is over, go cold turkey and stop the three way socializing with your DD, cousin, and other child. It's not ever going to go well and your DD will only end up frustrated and hurt. Don't explain it to DD. Just tell her it won't work it to see them. Start encouraging new friendships. Expect this to take a while and to be difficult. But this is very likely the only way your DD is going to make any growth. She is stuck in the same patterns with the same people.

You seem twisted and cruel. If kids have anxiety about friends and relationships, you don't sever those relationships. You find a way to muddle through. Cutting them off from their friends seems like it would compound the anxiety.

What is twisted and cruel is to keep insisting that her DD stick with this 3 way friendship that causes her DD stress and anxiety and constantly makes her feel like her cousin is better than her and she is in competition with her.

Anonymous wrote:Your child is not going to make progress in terms of social skills, social anxiety if she keeps trying to get this social situation the way she wants it to be. It's not going to happen.

I would not have the cousin attend the birthday party. Have her mom plan something special and then claim she forgot about it so now she can't attend the party..

After the party is over, go cold turkey and stop the three way socializing with your DD, cousin, and other child. It's not ever going to go well and your DD will only end up frustrated and hurt. Don't explain it to DD. Just tell her it won't work it to see them. Start encouraging new friendships. Expect this to take a while and to be difficult. But this is very likely the only way your DD is going to make any growth. She is stuck in the same patterns with the same people.

You seem twisted and cruel. If kids have anxiety about friends and relationships, you don't sever those relationships. You find a way to muddle through. Cutting them off from their friends seems like it would compound the anxiety.

Your child is not going to make progress in terms of social skills, social anxiety if she keeps trying to get this social situation the way she wants it to be. It's not going to happen.

I would not have the cousin attend the birthday party. Have her mom plan something special and then claim she forgot about it so now she can't attend the party..

After the party is over, go cold turkey and stop the three way socializing with your DD, cousin, and other child. It's not ever going to go well and your DD will only end up frustrated and hurt. Don't explain it to DD. Just tell her it won't work it to see them. Start encouraging new friendships. Expect this to take a while and to be difficult. But this is very likely the only way your DD is going to make any growth. She is stuck in the same patterns with the same people.

What is the familial relationship with the cousins parents? Do you think you could ask them to talk to cousin and make sure cousin pays attention to your DD?

Extremely close, cousin's mom is one of my best friends.

OP here. Is that a fair burden to put on an 8 year old who honestly hasn't done anything wrong?

I think it is ok to ask and not an unfair burden. But I think it may be hard for the cousin to keep in mind if she gets overexcited or distracted by the party. No harm in asking, though.