How about something like this:

“Because you’re a Palpatine? Honestly, I’m not even sure if that’s true.”
(Sits down next to Rey)
“But even if it is… some things are stronger than blood.”

That might be a bit on-the-nose, but I like how it shows that Force ghosts don’t have omniscient knowledge (otherwise Luke could just show up to call BS on the granddaughter story when they were heading to Endor), and conveys a message of “so what if you’re a Palpatine?”: the connections Rey has formed during her time in the Resistance are more powerful than any hypothetical blood relation.