To me it seemed odd she wasn’t his great-grand daughter. Like Anakin was easily young enough to be his son - and if she was younger than Kylo-Ren then the generational gap would likely be larger? I know its all sci-fi and aging seems to be all off in star wars anyway.
If he mentioned he once had a son (that had died) to Anakin in ROTS you could build some good hooks that would satisfy both Rey’s connection to him as well as strengthen his relationship to Anakin at the same time.