What is the Sharia ruling regarding a betrothed woman visiting her sister's husband's house and mixing with him, laughing, and joking with him? Is she sinful for that, and does her fiancé have the right to object to that?

The mixing of men and women and the resulting laughter and jesting are forbidden because they lead to temptation and the commission of indecencies. The wife's sister is a stranger to her sister's husband, so it is not permissible for him to look at her, be alone with her, or shake hands with her. She is to be treated like any other foreign man. A fiancée is a stranger to her fiancé, so it is not permissible for him to be alone with her or speak with her except within the confines of marriage-related topics and with the presence of a mahram.