Perhaps sensing that making it a shocking twist would be unwise, the revelation that Kylo Ren is Han and Leia's son is unfurled without much shock value fairly early on in the film by Snoke. We also see Ren look at the remains of the Darth Vader mask and refer to him as "grandfather", Han calls him by the name "Ben" late in the movie, and it's implied that Han and Leia split up after their son turned to the Dark Side. Also, Rey is most probably his cousin (but more on that in a moment).