Eventually Jyn can't take it anymore and tiptoes over, carefully brushing the back of her finger against Rey's cheek. She doesn't say anything, doesn't interrupt Cassian's soft lullaby, but she gestures back toward their room, gestures for him to follow her as she heads back.

Whatever she feels, whatever Cassian doesn't feel, it can't affect Rey and it is, Jyn is letting it because she's selfish and terrible and doesn't deserve the little girl tucked in Cassian's arms.

Maybe that's why she doesn't look back to even see if he's following her because she doesn't want to watch him not.