C.B.
Maybe she's not so much "favoring" the girl, but trying to step in and replace the mother figure for that child.
For example, she may have thought that you would have helped your son to make something for his dad for Father's Day and just wanted to make sure that the daughter also had a handmade gift for dad.
I don't think she's favoring, I think she's just trying to fill in for the absent mother. She knows that you're a good mom and that she doesn't need to fill those shoes for your husband's son.