
Angel is not a "normal" character, he is also a dominant omega. He has no problems kissing strangers. He kissed Koi (Connor Niles), who also is another dominant omega, when running into each other the first time.
Angel was kidnapped, sold and abused by 5 dominant alphas ( it needs 5 to controle a dominant omega, because they can use their pheromones against dominant alphas). Forced to bear them offspring, was not allowed to have contact to his children. When he tried to run away they physically hurt him.
So it's not surprising that Angel is not acting "normal".
And Keith is aware of it, so probably just goes along.

I read it some time ago, years ago aside from some side stories.
I remember that Keith is not allowed to have contact with Angel as a child, and he saw Angel was locked up and that they broke his legs. That I remember without re reading.
I think it was a also mentioned there that Angels is kept by 5 dominant alphas.
At least one of Keith half sibings (Hayden) makes an appearence in the side stories, not sure if another was only mentioned or appeared.
Angel appeared in the Lick me up if you can, the story of the Miller parents Ashley and Koi. There he ran away, was accused of sleeping around and when they caught him they hit him brutally.
The author also gave some information about the novel she planned for Angel. They mentioned that a family friend found out that he was a dominant omega, kidnapped him and sold him to 5 dominant alpha families. And that that novel would be about revenge.

If you don't mind of spoilers his is from Lick me up if you can, where Koi runs into Angel:
https://novellist.net/novel/lick-me-up-if-you-can-novel/chapter-198/
He is in the next 4 chapters.
God, I didnt remember this being dogshit, I came here to recall everything a I read to go to Kiss me if you can. Did I read to fast or does Keith literally kiss his fucking FATHER on the mouth??