After completing the Winter City dungeon, if you keep Etheryn in your party, she'll obviously be in the bar. But even when she isn't in the party and should be in Winter City, you'll still be able to interact with her from the bar.
I imagine if Arona is doing anything while the Champion isn't looking (aside from braiding Ryn's hair, apparently) it's probably 'wandering into the nearest outdoor areas, beating people up and fucking them sideways'. After Garth told her that if she ruined another of his beds he'd throw her out on her ass, Champion's alpha/bitch/drinking buddy (pick as appropriate) or no.
not like she has anything to be doing at the kervus camp anyway.
Judging by the dialogue if you're on the Chieftain!Arona path, Hretha probably prefers things that way. Much easier to keep things running when you don't have to pretend to take orders from your meathead of a little sister who's not particularly good at the job.