I'm all for it to be honest.
The mother is being kind and giving her daughter the chance to have a baby in the future if she wans to.
At least she's got the chance to use them if she wants and she wouldn't actually be it's biological mother (only in a birthing then nurturing sense).
When I read the thread title, I thought it was going to be a girl who was raped by her father and then decided to give birth to the baby - now that would be worrying.