I have a stepmom too. I do love her, but my dad has had a few girlfriends in the past after my mom passed away that I was never very fond of. I'm 24, but I understand a little bit about where you guys might be coming from. I thought maybe it would help if you got a perspective from someone somewhat in her shoes.
First off, I think that the things you do for her are so wonderful. My stepmom does those things for us too and she has always treated us like her own. So, she should be thankful and your husband should let her know that she needs to thank you for sending those cards. We all know that guys aren't very good at buying cards, let alone dropping them in the mailbox!
Do you ever call her? She might feel that the phone works both ways. Pick up the phone and give her a "just because" call every now and then.
When she comes home, maybe try to let her know what the family plans are. Let her choose what to make for dinner and maybe even ask if she wants to help or even make one of the sides to go with it. Go to the grocery store together to get the things you need. (Maybe even stop and get some dairy queen on the way home) She may feel like she should be treated like a guest because she lives in another state and isn't there everyday. But, instead of catering to her, let her be a part of it.
She's at the age now, where she might not want that third parent, but a friend. But whatever you do, DO NOT PUSH THAT! You'll only push her away! (Hear the experience talking there?) Just build that relationship and don't try too hard! Just ask her things, almost like you'd ask your friends. Is so and so still dating that guy? Have you seen that movie? (aka: if she hasn't you could rent it and all watch it together!) Maybe even let her know that you're always there for her when she needs that "mom" perspective but she doesn't want her parents to know.
I think that a friendship with your step-parent, when you're an adult child, is one of the most important things in that type of relationship. But, she needs to build on that. If she feels forced, it'll all go south. Just leave the door open for her to do the work.
I'm not sure how old her sisters are, but maybe let them send her some mail. Or if they're old enough to email, maybe they could even email her. Keep everyone in touch and it won't be so hard when she gets home.
I hope this helps!