They both have the same dad.. a fallen angel. The half-brother is more evil, hence the ‘demonic’ title I gave him for simplicity.
Basically the main character.. the teenager.. doesn’t know what he is.. just that he’s had powers he’s kept hidden his whole life. He exposes himself to save a little girl, which leads his half-brother to him. The teenager is a perfect hybrid between Angel and Human where as the half-brother is flawed, so he has resentment and wants to take his birthright of enslaving mankind.. the teenager basically learns all of this throughout the story and must over come both the half-brother and his own evil destiny.. basically a nature vs nurture story. Sins of the father infecting the son type thing..

“After a half-angel teenager publicly exposes his powers, he is hunted by his demonic half-brother who wishes to steal the birthright that the teenager knows nothing about.”