You have a need to know.
To alleviate the need, you could call them up and say something like:
Yes! I would love to celebrate her birthday with both of you! Thanks for inviting me!
Since we're on the subject of bdays, I note that my birthday is right before your anniversary and his birthday. Would we be planning and discussing celebration preferences for all these upcoming things at THIS date (her birthday celebration) or would we map that out at a later date?
While I'd like to know so I can get my calendar organized, I want to honor the birthday girl's preferences at THIS date since it is her special day. I always prefer to honor the bday person's preferences so I am really honored to be included in this year's bday fun! Thanks again.
Sometimes rather than what iffing a thing to pieces it is just easier to ASK. Set the precedent as "honoring the bday person and what their preferences are."
Not a precedent of "we all just celebrate together automatically."
Then see if you feel better.