What is the ruling on a wife and her sister suckling their respective daughters and the aforementioned son of theirs, in order to avoid the issue of khalwah (impermissible seclusion) with the daughters of the maternal aunt and the sons of the maternal aunt?

There is no harm in your wife suckling her niece, nor in her sister suckling your son. If the number of sucklings reaches five and this occurs within the first two years of the child's life, then the girl becomes a foster sister to all your children, and your son becomes a foster brother to all the children of his maternal aunt.

The suckling that establishes a prohibitive relationship (mahramiyyah) has two conditions:

1. That there be five sucklings or more, based on the hadith of Aisha: "Among what was revealed in the Qur'an was 'ten known sucklings that make marriage unlawful,' then they were abrogated by 'five known sucklings.'" 2. That it occurs within the first two years (the first two years of the child's life), based on the Almighty's saying: "two full years for whoever wishes to complete the nursing" and the Prophet's (peace be upon him) saying: "There is no suckling [that establishes mahramiyyah] except what rips the intestines."

Suckling children with the intention of establishing mahramiyyah is permissible and lawful, and necessity may even call for it.