Fitting everyone in

This is where open adoption can get convoluted... My daughter's birth-father has a serious girlfriend. My daughter hopes they'll marry. She asked me the other day what that would make the girlfriend when/if they do marry. "My step birth-mother?" she asked. Now me - I'm still trying to figure out what she could refer to my son's birth-mother as and I have no clue what to suggest for either. I need more terms of reference so I can sort out our family tree lol

Got any ideas please do tell me :)