I think there are two separate things going on.
Jill and Josy are becoming friends because they are both outgoing, charismatic people with shared interests. Fairly straightforward, really.
The relative lack of jealousy over the MC is mostly about out-of-game reasons: the story isn't at the point where the MC is supposed to commit to a girl, yet. Much like with the whole "no strings attached" business, it makes more or less sense depending on how you chose to play the MC.
That said, there are a few partial in-game explanations if we wanted to justify their reluctance to talk about it. First, the MC and Josy agreed to "no strings attached" so for Josy asking about Jill can only end badly; why open that can of worms now? Second, Jill is (theoretically) in the process of cutting the MC out of her life to placate Tybalt, so confronting Josy about the MC is at best making a bad situation even harder.
Third, while there is some awkward silence, it's not like the MC is making out with one in front of the other. Jill knows the MC must have made peace with Josy, but he spent most of his time in the library with Maya (and AFAIK, Jill doesn't know about Maya's relation with either). Josy likewise knows the MC had a date with Jill, but she doesn't know if it went particularly well and Jill seems disinterested in hanging out with the MC in the library. So at the moment the threat to each other seems low. The only potential red flag from each girls' perspective would be if the MC charms both in the D&G game. That's pretty low stakes, and we never see the two interact afterwards.
In the friendship path (with either M&J or Sage), Josy's response is pretty positive, IMHO. I agree she's either nervous or jealous when the MC is on both the M&J/fuck buddy paths.