this could spoil some interesting event story so don't read if you don't want this mystery spoiled.
it is made quite clear from the start that Nata is Natasha's father but she never says her mother's name and only the Wolfdogs get to meet Natasha. none of the Wolfdog girls made Natasha show any signs of one of them being her mom but Harpy gave away the biggest hints to who it could be. she steals a photo of Natasha's parents from her and it gets this reaction:
her mother is not one of the Wolfdogs and someone Harpy knows makes it very likely she is a Black Lamb:
this makes Yuri the one most likely to be Natasha's mom. why? cause Yuri sees Nata as a sensei and follows him to try to get stronger, Yuri has a good eye for seeing when someone is good so she knows Nata is nicer then he looks, and why would she like hanging out with him so much if she thought he was ugly. I highly doubt Sylvi is the mother cause she might agree to work with Nata but pretty sure she doesn't see a violent rule breaker like him as any of those things. so who thinks Yuri is Natasha's mother?