I am 16 years old and my sisiter is 14. i walked past her room on my way to the bathroom. her door was open and i looked in. she was facing me completely nude and was touching her "lower" area while looking down. i saw her chest and everythign else then she looked up and i was like omg. now i feel this wierd urge to masterbate thinking about her nakid. I wont do it because i know this is bad but is it normal for my mind to be thinking like that?