Do you think this is weird?

I'm 13 and a girl. My mum isn't home much. I see her maybe 3 times a week. When I do see her we don't talk just hello, goodbye. My older brother who's 22 is practacly raising me. The other day I had just got home from a friends house. I went upstairs to tell my brother that I was home. I knocked on his door but he didn't answer. I opened the door just to make sure he wasn't there. He was in his bed with his girlfriend naked. I was shocked and he saw me just standing there not knowing what to do. As soon as I could I turned and ran out. Things were weird when I was helping him with dinner that night. The next day he came up and started talking about "the talk" and about puberty. I already knew about puberty but the stuff in the talk I didn't because at school I don't do Ed because I do extended maths. My friend was saying in a random conversation that she never would talk about those things with her dad or a male. I told her about talking to my brother very open about everything. She said that it was very weird and strange. It it strange and weird? Also when I get my period how do I tell him about it because I will need stuff?