I found myself in a similar position a few years back.
I'm an only child, raised by my mother. When I was younger I stumbled on some information that my father had a wife and children when he was with my mum, making her his other woman. I never asked her about it.
Anyway, there's something high profile connected to the family, so a few years ago I found them via a facebook group connected to it.
I found I had a sister and two brothers from him. This was his family with his wife (now divorced) and also, another sister, who was actually a similar situation to me, same age as me and the other kids, clearly cheating all over the place!
Anyway, she had been somewhat welcomed into the family. The siblings seemed to call her sister and be in contact with her, as was my fathers sister. I'm not sure about him.
I fell into the facebook stalking too.
It was quite strange, it was like I couldn't look away, but also couldn't move. I never contacted them. Now I'm at peace with it. It wasn't what I needed right then. I think we're very different people, they don't seem to share my interests at all.
They live quite near, I've seen both of the brothers around the area.
I thought I was determined to contact them, but then I found that I had romanticised it, and realised that these shows that reunite families, afterwards they usually fade out and there's a fallout from it too.
I'm not quite ready for a life with them in it. I agree with PP, you're so brave to make a move and put yourself out there.