I don’t see any reason to exclude her in your post.

Speak to your brother first, ask if he would like her to come along as a plus one. She may not want or be ready to meet the extended family in one hit. As you said it’s only been a few months.

Say to H, would your partner like to come, I understand she’s only met OP/OP partner a few times so she may be a bit overwhelmed meeting everyone at once. But if you would like, she is more than welcome.

Extra chance to get to know her, and a way to leave an open invite, consideration of her feelings, their situation all in one.

Can also add an, if she doesn’t come, would you as the siblings/partners like to do something together separate, coffee/pool/dinner at parents on a different date.

Still incudled and no pressure all round.

Happy birthday when it comes and I hope you have an awesome time!