So I'm taking summer classes and there's this guy in my class who used to date my younger sister. The first few classes we only exchanged hellos and that's it. In like the third class he starts up a conversation with me and we just talk, at the end of class he says: I'll see you on Monday. Now Monday comes around and we talk like usual and it's very casual. He keeps staring at me too. And I ask him why he is staring and he just shrugs. Why did he randomly decide to start talking to me? Am I overreacting? Do you guys think he's interested? I told my sister and she's totally cool if we talk, she even said that he likes me.

P.s. I'm a little on the bigger side and he's really skinny, like basketball skinny