My family wanted to pray fajr together (I lead) but they got mad at me when I said women need to be behind the men (because we r all mahram). So I left them to pray by myself in fearing sin and my salah to be not valid. Is this correct and how do I make them understand?

Ustadh AbdulAziz al-Haqqan

The salah is valid, but this is hated and not the sunnah as you know.