Calling it now.....
The mystery female relative that Austin and Mason are working for is.....Tracy.
For the longest time they have been low-key setting up a plot where Austin has been keeping a secret with his cousin (?) Mason from Pautuk. Austin had to keep doing 'favors' at Mason's insistence, dropping lines about how "she" will be upset if Austin doesn't do what she wants. On the night Ava clubbed Nikolas, Mason was about to come over to Spoon Island to threaten Ava for some reason. This is why Austin came over--to warn her about Mason. Instead, he got involved in Ava trying to hide Nikolas and come up with a cover story, with apparently no sign of Mason. Now we see that Mason actually "saved" Nikolas---or at least moved comatose Nikolas someplace to get him hooked up to appropriate machines, etc. without telling Austin (or anyone else--except the person he was talking to on the phone).
So why do I think it's Tracy? Well number one, Jane Elliot is returning (yay!) for the 60th anniversary stories. Number two, she is a relative of Austin's---his aunt. Most importantly, I think she's been offscreen trying to collect the dirt on Victor Cassadine and his role in the "murder" of Luke last year. Now, with Nikolas in a secret location, she has a huge negotiating chip in the form of the Cassadine heir. She might have discovered that Victor actually had Luke locked up on Cassadine Island, and she plans to organize a "prisoner swap" of sorts. Victor knows that if Luke gets free and combines forces with all the others (Laura, Robert, Felicia, Anna, Mac, etc.) then he won't be able to finalize his efforts to re-activate Mikkos's Weather Machine, which has been his big, super-secret supervillain goal all this time. Would Victor choose his Weather Machine dreams over his nephew? Maybe newly-recast Nikolas would have to choose whether to stop his uncle's nonsense, or choose to go over to the dark side permanently?