Since were taking about biological.
I doubt that she is.
No information was disclosed that she ever was a mother aside from Karin's comment.
They never did answer the question and went around it.
Also, she never did answer why the DNA test was positive and whi the **** keeps an umbilical cord around and have it encased like a piece from the museum
Sakura herself told Sarada she was the mom so yes it was answered directly.
The DNA test tested positive because it was Sarada's umbilical cord that was used.
In Japan umbillical cords are often kept after birth. I guess either Sakura gave it to Karin as thanks for delivering the baby or Karin asked to keep it for some personal reason she has (probably to stay connected to the family).
But seriously the reason it was never addressed anymore than it was, was because it was a stupid red herring to begin with. It was obvious from the beginning that Sakura was the mom. Kishi put so many clues in the gaiden that you'd have to be blind not to see that Sakura is the mom. :wesobi: