If you are buying using a loan designated for owner occupied purchase, these special needs tenants must go, and you need to give them written notice in accordance with their lease. The seller also needs to give them written notice in accordance with the lease, and in accordance with tenant/landlord law in your area.
90 day closing risk? If the paperwork is not complete, there is always the chance that something can happen with the deal. What if the house is in different condition than now? What if the market value changes significantly? Ask your real estate lawyer for advice. NOLO.com might be a good place for Q&A.
That said, here are some questions:
Do they know the home is being sold?
When does the seller want to close?
Is the seller okay with a 90 day close?
Do they want to stay?
What is their funding source?
Can the seller teach you about sober homes?
Can you get a copy of the lease?
Did you know that special needs housing, where you rent out rooms separately but with shared kitchen and dining space, can (if you set it up correctly) provide greater income than renting to one household?
Look at all your options.
Blessings...