She'll be visiting here over the holidays, and I dread the possibility of repeating that unpleasant summer scene. I will need the kitchen more often than usual, and every inch of space in our small refrigerator. Furthermore, I do not want to support what seems to have developed into an obsession. I'd like to talk more with my stepdaughter (we used to get along fine), but she now considers me “the enemy,” and there are things I feel I can't say because I am not her mother. Making oneself at home is one thing, but I think it's presumptuous for a teenager to do whatever she pleases in a household. I want her to be here and be a part of this family, but the practical aspects of her visiting are very difficult, and I'd rather not have to "put my foot down." Her father tends to think I should go along with whatever she wants. Any suggestions for what I can do to help the situation? I don't need the moral philosophy, but I’d appreciate it if you could address the practical issues.
I am sorry you've had so much difficulty with your stepdaughter. The behavior you described sounds to be less associated with veganism and more with teenage rebellion. As painful as this may be for you, it is probably behavior you will have to endure until she matures a bit, especially because you are a noncustodial stepparent, which, unfortunately, often is a tough role to be in when teenagers are involved.
Although your husband is trying to be supportive of his daughter, it sounds as though he is neglecting your needs and the hurdles you are having in your relationship with her. At the same time, there may be difficulties she is having in her primary household or with her friends, and these problems may be following her when she comes to stay with you. It is a challenge to develop a trusting relationship with your stepdaughter when you see each other so infrequently, and she may be extremely uncomfortable and resentful of having to view you as an authority figure at this typically tough time of life. She may see you as responsible for the breakup of her parents' marriage or as a wedge in her relationship with her father.
To reestablish trust, make sure you have plenty of vegan food available for her--things she won't have to cook so she won't be in your way in the kitchen. Perhaps set aside a corner of the fridge for just these vegan items, and put them in a convenient front spot so she won't have to dig around the animal-based foods and get discouraged or annoyed. Try to stock the foods you know she enjoys (and let's hope she still likes them!). Do your best to be especially considerate and thoughtful, but don't compromise your own needs at the expense of hers.
When she arrives, you might want to take her aside nonchalantly to talk to her privately about your concerns, letting her know she is an important part of the family, how much you want her to participate in the festivities, and how you've made sure there's plenty of vegan food in the house so she won't feel left out or go hungry. You also could tell her how you felt about the events that transpired over the summer. Talk about your own feelings rather than point out how she disappointed you, as this will only make her angry and put her on the defensive. If you tell her about your own confusion, hurt, and sadness, there is a greater chance she will empathize with you and make an effort to understand. While you love her, care about her feelings, and respect her choices, she should know that you expect similar regard in return.
If it turns out that you simply can't get along no matter what you do, you both may have to agree to disagree for now, and attempt to tolerate each other as best as possible during her stay, even if that means keeping out of each other's way to prevent a blowup. Still, if you see each other just a few times a year, it would be worth it for all involved to make the visits as pleasant and peaceful as possible.
Do your best--that's all you can do. Luckily the teen years are temporary, although they sometimes can seem like an eternity for both young people and their parents.