Is it permissible to allow the maid to spend the night outside the home with her sister? Is preventing her from owning a private phone number considered a lack of kindness? Is it permissible to search her handbag upon her return?

The maid is a private employee, and her services are only owned during the period of hire. The employer does not have the right to prevent her from visiting her sister and staying with her outside of working hours, nor does your husband have the right to prevent her from owning her private phone. The default assumption is to consider a Muslim woman's affairs as innocent, and the noble verse: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy" forbids spying. Therefore, it is not permissible to search her bag. If any wrongdoing becomes apparent from her, it is obligatory to denounce it.