My mom was diagnosed with mild dementia but it is slowly progressing. She gets very sad or depressed more and more lately. Any suggestions on how to deal with the sadness?

My sis is so worried. My mom is about 86 yrs old, dad will be 90 in October. My sis is the principle caregiver as she lives near them (in NY, I'm in GA). She coughs incessantly & uses it not to leave the house. Her depression is scaring my sister (& me). My sister is taking her to a doctor on Monday. She won't talk to friends or us about it. We redirect her sadness by telling her everyone loves her, we need her, etc. My dad is getting affected by this as well.