Exploring the world, including the sexual organs, is natural for a child. What your five-year old is doing is thus not in any way unnatural. Think of dogs and cats, who very often sniff at each other's behinds. But in our culture, such behaviour is not accepted. Because of this, you need to set limits for her. Tell her in a gentle, non-anxious manner that she should not do this with her younger sister.
I have had many patients who have been subject to sexual abuse from elder siblings. They are often angry at their parents for not protecting them.
Do not let the children have a bath together. Do not let them sleep in the same room. Give the younger daughter long trousers which are not easy to take off. Try to keep the children in your sight, if possible. Do not let them be alone. Do all of this in a discreet way, so that you will not upset or embarrass your older girl.
It is important that the older girl gets stimulation and finds fun and meaningful things to do. If she goes to kindergarten, she can learn captivating things to do. You could buy a book about children's playing and activities, which can give you tips on how to keep your older daughter interested in meaningful and developmental activities.