@Ethan Cooke from Nolo:
" 90-Day Rule
The law limits rentals where the host is not present in the unit to a maximum of 90 days per year. Violators who continue to rent out their apartments beyond the 90 days are subject to a daily fine of $484 for first offenders up to $968 for repeat offenders.
“Hosted rentals”--rentals where the host is present in the unit--are not subject to this limit."
My interpretation (I'm not a lawyer) by in unit they mean the same dwelling unit. If the inlaw has a kitchen it is a separate unit. The goal of the airbnb law was to get more unit to stay in the rental pool so they probably don't exempt in-laws from the 90 day rule
If it is just for airbnb purposes guest may not need a full kitchen just a coffee maker, toaster, microwave that may be a way to skirt things?
Is the place not currently rent controlled? If it was built after the cutoff date I think it will be easier to condo convert but not my area of expertise.