I am pretty sure it has more to do with the relationship you have with the kids than when you joined the family...
My brothers Wife wasnt his wife when 3 of my kids came along but she is their Aunty.. ( or uncle if you ask DD lol) my sisters partner also wasnt there but he is in NO way my kids Uncle lol
My Dh's Nieces are my nieces too... I would never think that they were not my nieces just cause they are blood related to him iykwim.
I guess what im trying to say is that they are your Nieces/ nephews lol