Tough call. Tried it twice. I'm a bit young myself (24) so I wasn't sure how to deal with it. They never pushed their kids on me, but with one girl we went out to dinner w/ her daughter. I just felt like everyone's eyes were on us all the time. I felt like takin my placement and getting her daughter's crayon and writing,"This is not my child, nor my wife." I was younger when this happened so I blame it on being ignorant.
I would TRY (operative word here is TRY) if I was diggin her.
On a side note, my mom once said something that kinda hit home. When I was dating this girl my ma said, "well Freak Show, if you choose to get married down the road and have kids of your own, you have to understand that they will be so much more important to us." In that case it would be almost unfair for her daughter. Not that my folks would mistreat her, but its different cuz I'm there son and these kids came from me. My family and I have always thought 'long term'.