My own read on this is that none of the siblings are especially close, and that's by design.
Gothard has a whole playbook that's built around the idea of huge families and complete control over the children. If the children are close, they'll discuss things with each other that they don't discuss with their parents -- the seeds of rebellion can form. The entire family dynamic is built around a framework of snitches and tattling.
JB and J'chelle talk all the time about how "close" their children are, but I don't believe it for a minute. I think they just say it over and over, and the kids have no frame of reference to determine if it's true or not, so they just parrot back that they're "close" with no clue what that might mean.
The only people outside of her buddy group that Jill might have been allowed to be "close" to are Jana, Jessa, and Jinger. Jessa had dibs on Jinger and she seems like a mean girl who would get pleasure out of excluding Jill. I think that Jana's been completely emotionally shut down since she was first shipped off to JTTH more than 15 years ago, so I don't see much room for bonding there, either.
I agree with @Growsonwalls that all of the rest of the pack knows which way the wind is blowing at the TTH and aren't going to reach out to Jill if it will cause them ANY blowback at home.