Re: I could use some advice...
If you are close I would just chat about it in a pre-planned non-judgy way. Maybe she thinks since you are not having a part that you don't want a bunch of gifts or acknowledgement for birthdays and that you just handle them differently. Maybe if you don't have a part she has a hard time keeping track of when the birthdays are. I had a talk with my brother a couple years ago about how we would handle kids birthdays and I really think it's a good idea because now we are on the same page and know where the other one's POV is coming from. I think so long as you handle it delicately she will be understanding and you can clear up any issues. Keep in mind that you don't have to change her mind on anything...if she doesn't want to acknowledge the birthdays then you can't make her. But at least you will be able to talk with you DDs about what's going on after you figure it out.