The phrase "sisters are deeply in love" means that the feelings between sisters are predestined, which is a deep fate. It emphasizes the close relationship and interdependence between sisters, and can stand the test of time and all kinds of tests. It is a very precious emotion.

This statement may come from ancient literary works or from traditional proverbs, which express the feelings between sisters in a romantic and poetic way. In modern Chinese, this sentence is still used to describe the intimate relationship and deep feelings between sisters.