From what I was able to piece together, Lynette Kojima had met Kana and Kagome at some point after the twins had lost touch with MC, became intimate with them resulting in them being turned. According to the rules of vampire society, newly turned vampires have to leave their old families to help keep vampires hidden from the outside world. Apparently, Lynette, as the First Generation of the Kojima Clan, allowed the twins as newly created Second Generation members to make an informed decision on where to live. Miko and her mom have a similar clan situation.
If you pay attention when Mariko is telling MC that he can never sleep with (have sex) with Miko, she says that sleeping with a vampire ends only one of two ways. If the other person is female, a new vampire is made. If the other person is male, then a ghoul is made. Lynette giving the twins a choice is implied in how both of refer to Lynette and their biological parents.