In my head canon, Serana used vampire seduction to scramble the Dragonborn's brains into thinking taking her home was a good idea, and did the same to the Dawnguard when she came asking to talk to them. Sure, Isran is rude to her, but that he actually says: "Sure, just you wait there, un-attacked, un-imprisoned and completely un-harmed and let me get them for you" doesn't actually attack her to me indicates something has been screwing around with his mind.
But in all honesty, if the Dragonborn chooses to become a vampire it's just as stupid. The only way you get to find Serana and take her home is if you join an organisation dedicated to killing vampires. Literally overnight, the Dragonborn goes from: "I want to commit myself to the Dawnguard and eradicate vampires so much I sought them out on the basis of a rumour," to "Hell YES I want to be a vampire lord! I accept your gift, Lord Harkon" Huh?
I like Serana as a follower, but one of her lines is starting to get to me.
"That mud crab? It came from that river we're standing right next to, Serana."
"Where'd you come from?"
"Where'd you come from?"
"That Frost dragon? It came from the clear blue cloudless skies, Serana."
"Where'd you come from?"
"That Falmer I just shot? We've been sneaking up on him from behind for the last five minutes, Serana."
"Where'd you come from?"
"The Arch-Curate just unfroze them from their statued forms on either side of us, you blind dope!"