Is the intent behind adherence to the legal hijab in mixed family gatherings also to cover the face and hands, and is one sinful for attending such gatherings if some women do not adhere to this, imitating those who permitted their uncovering?

It is permissible to sit in mixed gatherings with women who unveil their faces and hands, provided that the Shariah regulations are observed, such as lowering the gaze. However, it is preferable and better for women to have their own separate gathering and for men to have their own separate gathering.