Good question. I'm not finding a direct answer to that. Handbook 1: 13.6.12 indicates that the parents of adopted children can't be updated until the adoption is final. I'm not sure where that leaves foster parents since it's hardly "final", but it is legal. Part of it may depend on if the parent's rights have been terminated or if there's a possibility that they might be returned.
It should be noted that Handbook 1: 3.6.2 indicates that it's possible for living children to be sealed to foster parents, but requires First Presidency approval.
I think this calls for a call to Local Unit Support to see if they can find more detailed guidance.
Unless something that changed, the membership system can only record "parents". It doesn't have the ability to record one than on set (such as biological, adopted, foster, etc.) like you find in FamilySearch.
My gut feeling is that it should be her biological parents until adopted. Mostly because that would flag future leaders as to the situation should they try to seal her to her foster parents.
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