What is the ruling on going to mixed beaches where 'awrat are exposed, and what is the ruling on hosting relatives who wish to go to them, and is that considered assisting them in sin?

It is not permissible to go to mixed beaches where 'awrat (parts of the body that should be covered) are exposed. It is also not permissible to assist those who wish to go there; rather, one must advise them and explain that it is forbidden. As for merely hosting them and offering them hospitality, that is permissible, unless your hosting encourages them to commit wrongdoing, in which case sheltering them would be a participation in sin.