The property has been sold-30 day notice given to mother-in-law. She is disabled and really needs more time to find suitable housing on her income. Her son and I live here to care for her and receive mail at this address. Is there any legal way for her to get an additional30 days?

ANY question on law needs to include your general locale as laws vary by area.

I'm assuming that your mother did not have a standing lease so you are month to month tenants. Without knowing where you are, how long you have lived there and the status of the lease, The only thing I can suggest is to try and negotiate with the new owners.

Were you not aware that the house was up for sale?

If you haven't moved out within the 30 day time and the owner files for eviction, at the trial you can request that the judge grant you a "hardship stay of eviction" for another 30 or 60 days in order to have more time to find a new place. There is no guarantee that the judge will do so but you can try.