What is the solution to the problem of not being able to fully relax with my fiancée due to her family and their arrogance, which is in contrast to her brother’s behavior with his niece?

Your fiancée is still a stranger to you unless the legal marriage contract has been concluded. Therefore, it is not permissible to touch her, be alone with her, or engage in any other forbidden actions. It is not permissible for your niece's fiancé to be alone with her. You must advise them and forbid them from this evil with wisdom and good admonition, and alert the girl's family to the impermissibility of her fiancé being alone with her.