Dating my moms half sisters brother?

Okay. My mom was adopted when she was little, and when I was about 14-15 she found her real parents. After finding out that, we found out she had half siblings. I'm currently living with her half sister, Kelly. Kelly's brother, mike has a major crush on me. We talk all the time, like alot of the same stuff... But I can't help thinking this is weird. So are we related or not before this goes any further... We're just talking but he keeps hinting that he wants to date me..