Master on Main means the the master suite is not up the stairs, second master wuld be even better since that means an en suite bath.
I have another listing with a second master suite. Call me at 541 207 5551 for a showing of NE Vinifera and SW Agate.
Will your grandmother be living in the same home or would she prefer her own living space (i.e. duplex, mother-in-law quarters, separate studio, etc). If not, how many bedrooms would meet your needs? Assuming that your family of four only needs 3 bedrooms, perhaps a search for a 4 bedroom house might be fitting. Some 4 bedroom homes have 2 master suites with attached bathrooms. Perhaps this would meet your criteria? I could do a search with the answers to these questions and provide alternaties as well. Areas considered and price cap or range would also be important. Have a great day!