Why do guys who like a "certain girl" hide their feelings when around her?

i just started working at the airport on friday. My manager Jheanelle and her little clique in the office told me yesterday that Jheanelle's brother Kris likes me a lot and that he's a fool over me. He mentioned my name to them, but I dont know exactly what he said about me. This morning, I was working with my sis and we got lost, so out of nowhere comes Kris and he's showing us around evrywhere. But he was mainly talkin to my sis while i was spaced out just lookin around the airport. After we sat down, he asked how old i was and wen i told him he started nodding his head and smiling. Then as we all started walking, i caught him staring at me and my sister started up a convo right when she saw that. Do you think he was giving hints? Flirting? Or just acting normally? If it's dropping hints, then why is he talking mainly to my sis?