*Have they said she can if she wants to, and they'd like it, or have they instructed DD that she is to call her mum. The former seems likely.
The two are very different.*
it's all there in the opening post...
Well, no, to be fair, we've got a 7yr olds recount. Even OP then goes on to say she needs to decipher what has happened.
I find it really unlikely that someone has strictly instructed a 7yr old to call someone who isn't her mother, her mother. It's much more likely that they've said something akin to "Once we get married, Jane will be like another Mummy"
Maybe the child said "so I call you Mummy now??" And they replied "You can". Which I don't think is necessary by the way. But if a little 7yr old calls Jane: Mummy, Mama, Jane, JJ, or Mrs (Bloody) Sparkles, at the end of the day, she knows who her mum is. And if a little nickname strengthens the bond between her and another woman who will care for her very closely, is it truly that disastrous?
Obviously if the unlikely event that "you must now call Jane, Mummy, or else" has happened, this is not ok.
Hope it all works out ok, OP.