Please forgive my question, I am new, and do not intend to offend. Xhex and Rhev are both sympaths, true. Xhex can go out in the sun, but Rhev can not, can this be explained?

Rhevenge's mother was a vampire so he had no genetic disposition for tolerance to the sun, but Xhex's mother is human (is/was) and she has a tolerance to the sun just like Beth.

I'm not sure but I think every half vampire/ half (insert race) gets different traits . For example both Beth and Butch are part vampire part human ( even though Butch went through a transformation ceremony they didn't eradicate all of his human DNA just activated enough vampire DNA for him to go through a transition) and Beth can got out in the sun but not Butch. I believe vamp/syphath works the same way.

As with Beth and Butch, they are half-breeds, but I'm taking a HUGE stab in the dark with my answer: Rhev's Mahmen is a Fallen Chosen. Chosens have serious mojo which makes him susceptible to most of all Vampiric ailments/traits/strengths. His Uncle was the King of the Sympaths, (I have to revisit "Lover Avenged" to be sure), & so he has some serious stuff oozing in his DNA. Now, Xhex's blood isn't as pure, though powerful. (My theory!) This is why she can handle sunlight, yet has the stealth of both a Sympath & a Vampire. She's also better able to assimilate in the Human and Vampire world than Rhev. (Notice how Rhev puts her out front? Ok, I'm 읽기 quite a bit into this, but this is my answer).