Serris,
I was thinking that his sister's name could sound kind of like Theta, but change the first letter on it, some what to look like one of these names like this maybe, who knows?
(Zheta, Kheta, Sheta, Fheta, Nheta!)
Maybe you could have Theta's sister a part of the secret CV underwater swimmer team, or in this case Blue, Gray or Brown Team. But when she and Ducky's Mother do have that brutal one on one fight to the death. They are moving all around/over the area, by fighting and stomping their through a small field of flowers until they end up knee or waist deep in water. In fact that's where Ducky's Mother ends up killing Theta's sister.