Also, given the context of the scene in which we found out what Emma’s connection to the Snow Queen actually is, it seems far more likely that she (the Snow Queen) was the matron of a group home, which would only make her a ‘foster mother’ for Emma in the most broad sense
Given the way the Snow Queen reacted to seeing Emma, and the image of Emma and the SQ in the shop all those years ago obviously heated, I’m going to guess that it’s more than just the broad sense. I think there was a real mother/daughter bond between them.
Emma and the Snow Queen having a mother/daughter-type relationship wouldn’t change the fact that the SQ appears to have been the matron of a group home for multiple children in the foster care system rather than someone who took Emma and/or one or two other kids into her own home and raised them in a traditional family environment for a while (which is what it would take for her to have been a ‘foster mother’ for Emma in the literal sense of the term).
Being the matron of a group home would “technically” make the Snow Queen a ‘foster mother’ for Emma, but the term wouldn’t carry the same meaning that is typically conveyed by its usage.