Why doesn't he tell me? Options

[karmakiller]

I've been trying to figure this out, so someone give me some insight, please.

My brother and I are fairly close. We'll hang out sometimes and talk to each other a lot. For some reason he will date girls and not even tell me about it. The girl who I'm assuming was his first girlfriend got along with me and she'd come over to see my brother (he works at home, because he took over the family business, but doesn't live at home) and her and I would talk and watch TV. The last girlfriend he had he dated for almost a year before I knew that they were actually a couple... that went horrible. I told him she was bad news, but didn't say anything else. She was bad news, and she caused a lot of problems between my brother and my family.

Then, on Sunday he had to go to the hospital and Monday I went to visit him and some girl is sitting there eating lunch with him. It was awkward.
Me: "Who are you? ...Did you graduate with him?"
Her: *awkward giggle "Um... I'mmmm... his girlfriend"
Me: "Oh, I haven't even heard your name until today."
My brother's friend: "Yeah, I'm lucky I even know who you are."

Then she said she had to go. This morning I called my brother's room and she was the one who answered. I still felt like an ass about yesterday.

Why does he not want me to know if he's seeing someone? His friend told me it's because the girl just graduated high school this month (my brother's almost 23.) I guess I just get a little offended that all his friends know and I am clueless. I feel like he's ashamed of me, because the two previous girls he dated never even knew he had a sister. :(