Hayley refers to Charlie & Harriet as her "future sisters-in-law", but since Harriet is married to Hugh, and Ajax is Hugh's son (but not Harriet's), Harriet would be her step mother-in-law. And since Charlie is married to Matt, who is Hugh's brother, she would be Hayley's aunt. Yes, Ajax was raised as Hugh and Matt's brother, but that was a façade that was dropped long since.