In the novels, I think it said that the werewolf-gene could only be passed down from father to son. Because I remember Leah saying she couldn't pass down her wolf-genes to her children, only males could. But then...how did Leah become a werewolf? I really don't know...probably because it's extremely rare and therefore the reason why Harry Clearwater had a heart attack when he saw his daughter actually turn into a werewolf like her brother. But...that's my guess...

Leah was an exception. Generaly no girl or woman becomes werewolf, just men. First, as sapherequeen pointed out, the genes are passed down from father to son, not dother.
And second, in the book it says that Leah actualy can't have kids. So its better that his sisters don't transform...