Doesn't Naomi and Annie share a brother? What happened to that storyline ?

Well in the first season they found there half brother but it turned out he wasn't there real brother, i guess the writers didn't finish the storyline, thats one thing i don't like about 90210, they leave too many important story lines.

Yes, Naomi and Annie share a half brother. That storyline is back...4 seasons later. This time Naomi has successfully found their REAL half brother. The half brother's name is Mark, and he first appears in 5x14.