J.R.
Hi there,
I used to be in your 9-year-old's position. As a child of divorce it can very difficult to feel like you "fit" completely on either side of your family, especially when both sides re-marry and start new families.
I remember having hurt feelings when my family would do things without me. Yes, I understood that life couldn't possibly stop when I was gone -- but when major things happened and I couldn't go because of a custody arrangement -- especially when no one had even asked my opinion on what I would have liked to have done -- I ended up with hurt feelings. I read some of the other posts -- I actually don't think she's being immature or adjusting or acting out for attention. I think she really has hurt feelings. If she lives with you during the week she thinks of you as her "main" family.
Here's a good rule of thumb for you: if one of your biological kids had a prior commitment and couldn't go along, would you try your best to work around them? If so, you should be doing the same for your stepdaughter -- even thought it's obviously more complicated because it is every other weekend and not once in a while. No, you can't always ONLY do fun things on every other weekend, but you should be working very hard to accommodate big things -- like birthday parties -- so she can be involved, otherwise it makes her feel like... a stepkid! Yuck!
So, no, it's not reasonable to ask your family to rearrange a party schedule for her, but I think the most obvious solution is to call her mom and tell her you have realized your stepdaughter's feelings are getting very hurt when she misses big things like this birthday party, and see if bio-mom is willing to be flexible with weekend visits. Sometimes she may have to go to her mom's 3 weekends in a row and then miss a few weekends so she can be included in things you do as a family. It's not that you (or your stepchild) are choosing doing something fun over seeing her biomom -- it's that you are making sure she is secure in her role in your family. If she is excluded from an event just because she has to go to her mom's house, it is just making her feel more like a stepchild than she already feels just by nature of being a stepchild.
The baptism is complicated because it means she absolutely has to go to mom's, right? Can you sit down and tell her you know she is sad that she has to miss this trip, but that you're sad she has too miss it too? Just to validate her feelings? Once my parents made me a video when we were in a similar situation - I had to miss the family Christmas party. Everyone said something nice about me and that they missed me and showed it to me when they got back. It meant SO much to me even though I acted like I didn't care. =) Maybe you can do something like that for her.
The best divorced/co-parent situations I know of where the parents work together all the time, even through gritted teeth, to make sure their child knows they are loved on all sides. It can be done and be done well. I think you and her bio-mom need to do whatever you can to communicate more and plan around making sure she can be at your house when she needs to be and at mom's house when she needs to be to be included in as much as possible. Could she go to mom's one night during the week instead if she was going to miss a weekend visit? (Hopefully her bio-mom can be level-headed!)
I have a good relationship with my stepmom after all these years, but they still have a professional family picture up in their house from a weekend that I wasn't there -- so obviously I wasn't in the picture. That picture STILL bugs me 25 years later. =)
It sounds like you really love your stepdaughter. Just remember that she probably needs extra assurance that you and your husband love her as much as you love your other kids.
I'd give the video thing a shot. Hang in there mama!