Nicky was either abandoned or given to the church as a child, he was never really able to build a mother-son relationship with anyone since he was raised by churchmen. He can't help but crave that kind of closeness sometimes. Adding to that the fandom headcanon that has Andy looking at Nicky as the little brother/son she never had (I'm angling more towards the latter).

I'd prefer this to be non-sexual. Maybe some misunderstandings at first. Like the first time Andy disrobed in front of Nicky and he blushed and then she repeatedly caught him looking pensively at her breasts... but once they start doing it, it's simply because it gives them comfort. Joe knows about it and honestly thinks it's cute up to the potential!author whether the rest of team knows as well.
Although I would love a scene post-London when they both need the comfort of the act and Nile ends up seeing it, cue Joe having to explain it to her.

Also up to potential!author whether Andy actually lactates or not during the nursing, I don't mind either way.