There is NOTHING about being "just" a step-parent and there it is no way being "lesser". Being a step-parent is one of the hardest roles to play in the world and you should be proud to stand next to your step-daughter while she cuts the cake, etc. and not let anyone treat you as less. It takes more than a blood connection to be a mom and you doing all that you do to make sure she is happy on her birthday shows that you have IT and so much more.
I applaud you in doing all that you describe that you are doing, but I think that in backing off during these family events, you may be adding to your SD's confusion about your role in her life. If you are backing down and letting your hubby, MIL, and SD's mom treat you like a "lesser" parent, she will see this and therefore interpret you as such and in the end possibly respect you less. So I say, take step up and step into the family circle and let all those people know that you aren't there just as a pretty decoration, you are there for that little girl.
Additionally, if your hubby also treats you as a "lesser" parent because you didn't birth his little girl, I think it's time to sit him down for a little talk about things. That is disrespecting you. When you married him, it is because he also thought you would be a good mom to his daughter, not just a temporary stand-in for the female role. I understand SD's mom treating you like that, but your husband and his mother should not be treating you like that. You have stepped into the biggest role of your life and they have no reason to look down on you because of the lack of blood ties.
Now that we live so far apart,mMy step-son has a separate party at his mom's house and at our's, but in the beginning we would do a combined party. His mom would promise to pay us half of everything for years but never did, so now this works out better for all.
On a better note...enjoy the birthday party and hope the weather works well for it and don't feel bad if SD doesn't pay attention to anyone but her friends. That's the joys of being a kid, you can be as selfish as you want without getting into trouble. LOL Good luck and God bless.