The Elder Scrolls: It's the IX Divines, you milk-drinkers

[Kareeah_Indaga]

but (outside of alignment) there's no logical reason for the Dhovakiin to not leave Serana dead in Dimhollow and take the scroll back to Isran herself. Or if you decide you don't want to murder "innocent" vampires, I've yet to see any reason why you shouldn't let Isran deal with her at Ft. Dawnguard.

^This. It annoys me so much every time I play the Dawnguard quest.

The trip into the soul cairn is the only part of the DG quest chain would break, and I'm sure there would be some way to incorporate that w/o Serana if Bethesda had really wanted to.

The simplest way to get past the 'must be a relative of Valerica to enter' part would be to take blood from Serana's corpse. Or, did anyone say it has to be an immediate relative's blood? Have a sidequest wherein Isran has you hunt down a book of Valerica's family tree and find a many-times-great-grand child of a niece of hers or something. Then persuade that person to give you some of their blood via Speech check/quest.