My stepsister uses my shampoo and conditioner and I'm going to ask her not to.
Do you have reason to think that is is knowingly using your stuff?
She knew that they were mine.
Your stepsister should have asked your permission before using your shampoo and conditioner. I get why you're upset.
It's not like they're expensive or anything, but she should have asked first.
It's great for family members to share things, but one should ask before using another's things. Regardless of price it feels crummy when people take something belongs to you. Kindly ask your stepsister to ask before using your stuff in the future.