My friends little sister has a crush on me, what should I do?

Anonymous
Ok, so a couple weeks ago i went over to my friends house to watch a movie. After we were done with the movie his little sister came and asked who i was. I then introduced myself and we talked for a bit. After, i went over to my friends room to play some video games and then i had to leave. My friends little sister constantly looked in the room and left. When i was packing my things up to leave she was outside the door of the room and kept looking at me. When i was done packing she quickly ran upstairs with her friend. After that i left and said my goodbyes. Now, yesterday i went over to my friends house again since the last time. This time however i was holding a tournament at my friends house. She was lying on one of the couches and when she saw me she immediately said my name and greeted me. I responded back and talked for a bit. After she moved to another couch and she took her phone out. I am almost certain that she was taking pictures of me/recording video. I was wearing shades and while i was walking to the bathroom she was turning her phone in my direction. She kept pointing her phone wherever i went throughout the day. She then left somewhere and didn't see her again. After the tournament was done, i found her instagram and was surprised! On her description she had my name in capital letters followed by two hearts. I am pretty sure she likes me however there is something that i am debating. I am 16 and she is 13. I find her cute but what should i do?
My friends little sister has a crush on me, what should I do?
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