Tenant Bringing in Additional Family Member Advice

My parents currently rent out their basement to help supplement their income and are located in Mississauga.

In July we rented out the legal basement and had the contract signed by a brother and sister from India. The sister is now looking to bring her husband over (suspect no income) since the sister is a student. The brother passed the income verification and for the last 3 months have had no issues.

I know as a landlord I have no legal right to say no to bringing an additional tenant. But are there best practices that the community typically does during situations like this?

I think at the very least we want the tenants to re-sign the contract with the same terms. Just so I can have his name on the contract and have the ability to do some form of security check by seeing passport etc.

thoughts? What have others done in ON?