South
Africa is well endowed with
Wilhelmina people, most of them elderly and all of them Afrikaans. So, now I come to think about it, yes, with an English pronunciation it could work well - just, not here! And your elder daughter has a fem form of a male name, come to think of it, so it would be a link between Sisters 1 and 3.
Billie would be delightful as a nn, but in my experience, nns take care of themselves in their own time.
Willa is good too, but there are so many little boys answering to
Will that it might be worth avoiding. Or not! I don't know much about
Oz naming.
How would
Wilhelmina work with your mn possibilities?