I do believe that Issa is her father, I think Issa just mates with all the strongest vampires from around the world without the connotation of marriage. Wanting powerful off-spring is probably all he wanted to fulfill his ambitions. What kinda begs to question, why haven't he had any sons? I kinda feel bad for him, not being able to have a boy
daughters are nice and all, but as a man I would like to have at least one son to bond with