The name "Rebesther" is the combination of the queen of the vampires: Rebekah, and the most powerful witch: Esther.
Reb = Rebekah
e = both names (Reb "e" kah - "E" sther)
sther = Esther
For me. this name sounds to be used for both genders, as the names of Michael, Alex, Jessie, Redd, etc.
What to say about this, Rebesther is a neutral name or just limited to female name?
Note: Imagine it's the first time you hear, do not take into account the characters of the series, and the writers decided to create a new character, as Klaus, Phillipe, Adrienne, James, Amy, etc.