What is the ruling regarding my wife's relationship with her paternal cousin and his children and wives? Is it permissible for her paternal cousin to be in seclusion with my wife, knowing that my wife was suckled by the paternal cousin's first wife?

The husband's paternal cousin is considered a father to his wife if she was suckled by his wife, and thereby all his children and those suckled by his wife become her siblings. It is permissible for him to be in seclusion with her unless there is suspicion, due to the Prophet's (peace be upon him) saying: "What is prohibited by lineage is also prohibited by suckling."