in lover avenged, it is revealed that the princess in rhev's half sister...really? by blood or as a fellow symphath? he's going through enough hell without adding incest to the mix.

yes they share a symphath father

they have the same father.
At the time of the rape, Rhev's father was King.
Which is why he is a prince and the King

They are brother and sister by blood through their father who was the Sympath king. Although the princess is full sympath Rhev is half vampire through his mother who was raped by the king.

Rehv and the Princess share the same father, Rehv's mother, a fallen chosen, was raped before she married Bella's abusive father. This added dynamic, (incest) helped make Rehv sacrifice for his family that much greater.