Wei Wuxian has suspicions and he is doing his level best not to let himself think about them too much, because Jin Ling is his nephew and he loves him.

(And he's going to marry Yuan whether he likes it or not, as long as that's what Yuan wants)

"I need to talk to you." he says simply "About Yuan, so keep your voice down and let him sleep."

He needs to talk to both of them, technically, but he thinks this might be easier if he explains things to Jin Ling first, and gets him on side.