Masturbating with a pair of sister's panties

One day while I was home alone I was masturbating with a pair of my sister's panties. I came and left a few drops of cum on the crotch of them.

Maybe half an hour passed and she got home. She opened her drawer and pulled the same pair of panties out that I had came in. I left her room so she could change. When she came out she was in her panties as her jeans were in the dryer and I could see a wet spot on her crotch right where I came, luckily she is on birth control.

Knowing that my sperm was now in her pussy made me so horny that I ejaculated in my pants. When she left I took out all of her panties, clean and dirty, lay down on my bed, and began to masturbate with panties all around me.

It took me a few times but I successfully left my cum in all of her panties. I returned them all to their original places except for a pair that had been in her laundry basket which I kept for myself. Now every week I refill her panties with my cum.