I'm going to splash my sister with water
Can I ask you not to splash her with water?
Sure, but that doesn't mean I'll listen.
Your sister probably doesn't want to get wet, so it's not nice to splash her with water. She may even end up getting sick if it's cold out.
She's not responsibility even if she falls sick.
It's not nice to treat your sister in this way. She's your family and you should care for her. Being nice always pays off.