Due to the force/ancient sith artifacts/cloning or other convenient plot point Kylo manages to bring back Anakin, specifically Anakin post 'killed the younglings' pre lava.

+Anakin is told Padme is dead, Kylo comforts him
+Kylo doesn't mention the grandpa thing, he just says he's looking for a new master and Anakin was the greatest sith ever
+Kylo goes full creep and attempts to keep Anakin as his
++dirty wrong sex

Up to author whether or not Anakin realizes the person who brought him back to life is his grandson at some point and if so when