The father is the same for all the siblings. Aikokiri
No, it's not her father. I'm guessing she's the réincarnation of the woman hé loved (and for who he's keeping her while not even knowing that she existed before) She's just leverage to blackmail her "mother", that's all
I can't wait for the youngest of those brothers to finally get his, because so far FL is kissing everyone except the one who's really in love with her