Hi, I'm 19 and recently spent a week on holiday with Ellie my 12 year old neice, she isn't really a blood relative but I consider her my neice as I have a compilcated family situation. I have spent a lot of time with Ellie as she has grown up and she will be 13 in a months time. I have been dealing with difficult feelings in my mind as I am slightly fixated on her now developing body. I find myself aroused in her presence and particularly on holiday one day we were along at the pool and she was spending a great deal of time standing in the spray park where jets of water shoot out from the follr straight up. She had positioned her crotch right over a jet and the look on her face said it all, plus her obviously perking nipples burning through her swimsuit and a swollen engorged labia. In the pool she spent most of the time hanging off the side with her crotch pressed against the water jets in an obvious state of arousal, I am battling with the sensations running through my own thighs as I watched her, I also found some pleasure in checking her underwear and smelling some of them-help
You should really stay away from your niece until you can get a handle on your own arousal. There were probably oodles of girls at that water park, most of whom were much more appropriate for you to fantasize about than your niece. You don't seem to be even a little bit worried about your niece and that she is masturbating publicly and using a dysfunctional method.
Your approach to this situation seems to be to stare at her until you can't take it anymore and then go someplace to masturbate. A much more productive way of handling this would be to:
1) Look away; respect your niece's privacy
2) Let one of her parents know that she's masturbating publicly and that she's using pool jets to do it
3) Look around and see who else might be staring at her
4) Possibly telling your niece herself that you saw what she was doing and other people could see as well. You might also tell her that using running water to masturbate can be harmful to a girl's self-discovery.
And leave her underwear alone! Treat her with the same respect you would want to be treated with.