Why do post-90s girls add the word "ha" after every sentence? What does that "ha" mean? Could it be between brother and sister?

This "ha" is used as a modal particle here and does not participate in the semantic expression in language formation. Generally speaking, the object of application is people who are not much different from themselves, or the use of such modal particles is mainly to highlight the user's liveliness, playfulness and cuteness. They belong to modern popular modal particles and are mostly used by young and fashionable women. It may have other meaning in some dialects, but it is not known yet.