I feel like that’s covered well enough by his following line, perhaps with a tweak:

“What are you most afraid of?”
“Myself…”
“Because you’re a Palpatine? Some things are stronger than blood.”
“Final lesson. Rey… Confronting fear is the destiny of a Jedi. Your destiny, if you don’t overcome Palpatine, will mean the end of the Jedi, and the light. Palpatine is beyond redemption - he must be destroyed. There’s something my sister would want you to have.”

(Though I don’t know if any of this is said onscreen by Luke, in which case this may be a non-starter. Still, I think it’s clear enough from the second part that he’s directing her to do that.)