Look, I didn't want to be the first one to say it, but... I think the only solution to your problem is masturbating to the thought of your sister in her swimming suit. Imagine how each portion of smooth skin is bunched up underneath, hardly able to breath, waiting to be released from that nylon cocoon. Smile at her innocently while she takes it all off right before your eyes. Walk slowly over to her. Kiss her. Grab her. Cry because it isn't real.