This my assumption but Ethan sucked blood from his father's prev lover Ira, the pure blood one , also remember before Ethan went to see Tristan uncle he gave a vial of his blood to Tristan and while doing the deed Tristan sucked Ethan blood so they were blood bound, and at the end he gave his blood to Ethan , (I remember also the pureblood vampire forgot his name that was in love with ethans dad told him he could become pureblood if he sucked him.dry almost at killing point but not quite ) I just assumed that happened cuZ Tristan had that purebloods blood and ethans blood within him.
Idk can't say it's true or not, guess we gotta wait for sides or something

What's up with that freaking ending.. So wtf , so ethan is on the verge of dying and Tristan gives him hims blood, sooo at the end ethan lives and Tristan becomes a vampire .... That's what I could gather, and what's up with Wren???? Fuck so many things left unsaid tf