let in on
let somebody in on something | let somebody into something
phrasal verb
(informal) to allow somebody to share a
secret
Are you going to let them in on your plans?
e.g.
I'm lettin' you in on this because I like you. あんたの事気に入ったから話すんだぜ (
/yupeco/『ゴールデンカムイ』
1話)