You follow the lease until and unless the son or granddaughter produce either a power of attorney from or guardianship papers over the grandfather showing they have the legal authority to act on his behalf. And if she again tells you to not contract him, tell her that you legally have to until they show legal authority to act for him. You can and most likely should copy the granddaughter and son on any communications (which should be in writing, sent some way you can prove delivery)--that courtesy costs you nothng. But make sure that you always send notice to the grandfather (again, in writing sent some way you can prove delivery), since you must comply with the lease.