Rental Lease, tenants over 18

[Wakes Andre]

Should everybody who lives at the property and over 18 signs the rental lease?

Ex.:

At the beginning of the year, my tenant reached out to me and asked if her brother can stay with them for a few months. I had no issues with that and we've made an addendum to the current lease.

Now, the current lease is about to expire, and her relative is going to stay with them during the new lease timeframe.

Should I add him as a co-signer on the lease or he can be listed as an occupant but not financially responsible one? Please let me know the possible pros and cons.