Is it forbidden for the fiancée to sit with her fiancé's non-mahram relatives, and what is the solution if this causes a rift between the fiancé and his family?

The mixing of non-mahram (definition) men and women is a reprehensible act, and its harms are numerous. Safety lies in avoiding these mixed family gatherings. The matter should be addressed with wisdom, gentleness, and kindness, along with an explanation of the Sharia (definition) ruling. If they refuse and do not accept, it is not permissible to appease them at the expense of religion. Recall the saying of the Prophet, peace and blessings be upon him: "Whoever seeks Allah's pleasure by displeasing people, Allah will suffice him against the burden of people. And whoever seeks people's pleasure by displeasing Allah, Allah will entrust him to people." You should be patient with them and keen on maintaining ties with them, even if they cut you off, in fulfillment of the Almighty's saying: "And not equal are the good deed and the bad. Repel [evil] with that which is better; then behold, he between whom and you was enmity [will become] as though he was a devoted friend."