I want to touch my sister while she sleeps
Why would you want to touch her as she sleeps?
I would like to feel how soft her body is.
You shouldn't touch your sibling at all especially without their consent. It is immorally and criminally wrong to touch someone while they are sleeping.
It's okay as long as I don't tell her.
Even if she never finds out, it's still wrong to do. You should never even think about touching your sister like that.