Our society has a real problem when it comes to the bodily autonomy of children. While your actions may be considered abusive, you are not doing anything that hasn’t been repeatedly modeled to you and condoned by a society that says that children essentially have no rights of their own.
What you can do to stop is to remind yourself that your little sister DOES have bodily autonomy and rights, and that she can express her opinion and preferences.
Ask your sister if its okay to tickle her or give her a hug. If she says ‘no’ respect that that’s her choice.
By reaffirming that she has a choice on whether or not she wants to be touched, you will be helping her start to build good, healthy boundaries and avoid a lot of the problems that women are socialized to have in our society where we are taught to believe that our bodies are essentially public property.