Sorry, I have no real advice, but there's a specific Step-parenting topic, which might be more useful because other step-parents must at some point experience something similar. They will probably be able to advise you.
From my limited knowledge, I'd say 8 is young, and an age when they are easily pressured. They can also still believe that their mum and dad will get together again- she probably wants that. If you've already told her you're getting married soon/are engaged (congratulations) then this might be her trying to stop it going ahead in an attempt to get her non-step parents (you and her mother) back together. Alternatively, she feels she can't say she's happy or enjoying herself to her mother because it feels like she's betraying her mother by acknowledging that you both can make her happy and that she loves you, because it feels like she's being a traitor to her maternal mother. I'm guessing that will be more likely if it's a particularly heated and venomous divorce.
Well done for you and your fiancee for trying to be fair to your daughter as well as your step-daughter, and I can see how hard it must be, for your fiancee and for you.
Try and show that you 'forgive' her, but talk to her alone (in her bedroom or somewhere private) about how your fiancee is heartbroken about it and thought she was happy. Don't go on about how your fiancee treats her like a daughter and loves her (I'm sure she does, but this will sound more like a lecture than a conversation, and will make her defensive).
Ask her what she doesn't like about the Christmas you had, what changes she wanted to make, why she didn't like it, other things she doesn't like etc; Even though they might be groundless/reasonless/not really possible, try and show you are listening to her.
Ask your fiancee if she can come in and talk to her at that point after you've discussed it (I think the first part should be private as its discussing problems and your daughter will feel better about it) but don't get your daughter to apologise as such. Make sure you make no mention of being cross with her or punishing her or anything- this will make her dislike your fiancee more (she'll probably feel embarrassed about it, which might come across as defensiveness, agression or anger as well, and see your fiancee witnessing it and dislike her for it), but make sure to talk to your daughter.
Sorry for being a bit useless!