I still think some of these elders need to be replaced.
Now as for Ariana's sister... Hopefully she'll say that she's impressed with Arc's skills, and she'll thank him for keeping Ariana safe; and that's it... Well maybe also talk about the main enemy and whether Arc has a connection to the guy. Nothing more than that, especially if just her threatening Arc, dismissing him, and/or saying that he's not good enough for Ariana.