In our house when I was growing up, it is pretty common for us to walk around in our underwear and a T-shirt in the morning when we first get out of bed. I have a sister that is a couple of years younger. Once she started developing, I had to tell myself to not look at her too much. So it wouldn't seem obvious anyway. The sight of her in her underwear and a T-shirt aroused me and I would often masturbate thinking of her.
It was the summer before my senior year in high school. Both my parents worked, so it would just be my sister and me. This one particular morning I was in the kitchen eating breakfast when my sister walks in wearing a T-shirt that was pretty tight and her panties that were pretty tiny also. I noticed her nipples were hard. She walks past me to the fridge. She opens the fridge and bends over looking for something. This is making me aroused. As she is bent over she looks back at me and sees me checking out her ass. She just smiles and goes back to looking in the fridge. I am quite aroused by now. I decide to eat my breakfast quickly and head for the pool.
I make it to the pool and start to casually do laps. Before long my sister comes out. She has on her swim suit that is from the year before. She has grown since then, so it is tight and I notice she has a camel toe going on. This isn't or is helping my arousal, depending on how you look at it. She walks on the diving board and dives in the water. She immediately climbs out of the pool. Her swim suit is white and blue striped. Since it is from the previous year, it is worn and you could see through the white stripes since it is wet. Her ass cheeks were hanging out. Her nipples are still hard. Once she is out of the pool she goes to the diving board again and dives in. She does this multiple times. My dick is hard as could be. She then gets out of the pool and lays down on her stomach to sunbathe right where you get out of the pool. I was thinking my dick isn't going to go down unless I stroke it.
I decide to get out of the pool and go to my room to take care of myself. As I get out of the pool I get a great shot of my sisters crotch. She is still laying on her back with her legs spread apart and knees bent with her feet in the air. As I get out of the pool, my swim trunks suck to my body and there is no hiding my hard on. I am hoping my sister doesn't look around at me and notice. But she does and she looks straight at my dick.
I go straight to my room. Peel off my swim trunks and start stroking right away. Soon, my sister walks in my room and sees what I am doing. I stop. She walks right up to me. Presses her body against me and says 'I'll give you a hand'. She starts stroking me. I tell her to tighten her grip a little. She does. I shoot my load soon after that. Sis then takes her swim suit off, lays on my bed, and starts to play with her pussy. I get hard again and start stroking my dick. She brings herself to orgasm and I shoot another load.
We stayed naked the rest of that day till our parents came home. We were both disappointed when school started again. We seldom had time to spend alone with each other. At the end of the following summer, she told me she hopes I come back for summers after I go away to college. I definitely did that. She ended up setting me up with a friend of hers. We no longer have our alone time with each other. Sometimes I think she always wanted me around, so she did what she could to make me happy.