Story-wise because the premise is her progeny are half-human half other-wordly. This mix is somewhat vampiric as they feed off each others strengths through their physical interactions which are violent in nature, at least where the male is concerned. So much so that his natural act of sex is deadly to a human, who cannot use his inseminated infusion to recover. Likewise, if say a predatory human male were to attack one of the sisters in a sexual way, she would drain the life from them in a hideous way.
Practically speaking, only Hallmark movies do not have a tragic component, and I submit this is not the Hallmark channel. This plot could continue, but for now I am running out of story and I don't want to make it any crappier than it is.