I like my little cousin's mate is it normal

When I was 16 I was in high school with my little cousin (13) and her mate (same age) I started to like my cousin's mate. It was a when we where playing football together with a couple of other people I can't explain it but the way she was moving it made me see her in a "different light" i did not really see her like that before till that moment it shocked me. She's my cousin's best mate if I said that I liked her I don't know what I would have done. Is this normal for you to like you little cousin's best mate I would really like some help.