I'm confused by that last chapter? Are they related by blood or not?

from what i gathered: no. they are related by law. alpha's father married a woman who already had a kid so the kid became the father's step daughter. the woman died, leaving the daughter and the father remarried, his second wife gave birth to the alpha.

the daughter grew up and had an omega son. the omega grew up and got into a relationship with his step uncle?

i think that's the case? and the reason why she feels so disgusted is because she basically grew up alongside the alpha and they are step siblings