Current tenant is sibling of tenant on lease

[Krisleigh Hoermann]

New landlord here! I recently acquired a multi-use property - half retail and half residential for the business owner. I recently discovered our tenant is lending the residential portion to his sibling. I'm OK with it (he has been a long-term tenant of the property), but want to make sure we're covered too. He's currently on a month-to-month lease and won't sign a new one without the first right of refusal. Should I send a letter stating that I am aware of the change in residence and his sibling is bound by the same terms? Do we need to ask his brother to sign a sublease? Any other thoughts?