A part of me wants to believe Luke doesn't have a kid cause of what happened before. And, if Ren is his, who's the mother?
A part of me doesn't want to think Ren is Leia's kid cause Han would have reacted differently, I tend to believe.
With Luke and Leia's reactions though... there is some history with Ren. Although, why would Leia not go see Luke unless she already knew where he was to begin with? Maybe Leia does know where Luke is and they communicate with the Force.
Or, maybe R2D2 knows where Luke is and when he needs to be found, R2D2 wakes up. Maybe Luke is able to communicate with R2 as well.
However, in the end, she is related to the Skywalker line. In the trailer, Luke doesn't say, "my daughter has it" nor does he say "my niece has it". But, in the end, Han and Leia had a reaction to Ren as an uncle or aunt would. Luke's reaction, as a father, is unreadable. So, while Luke may be her father, there is no confirmation to it. However, in the books, Luke does name his kid Ben so Leia screwed that one up. We just need Mara Jade now.