Talk and talk to her. Ask her what she thinks would help her to feel better.

She may have adusted to losing your dad, but then her own advancing age may be producing separate issues to deal with. We all need to feel valued at all ages of our lives and she may be struggling with that.

Is there something she does as a hobby that she could do with a group? Or that she could do to donate somewhere, an acitivity that would make her feel useful and valued?

Would she be willing to try a group for people who've lost spouses? A grief support group of some kind could help get her talking and give her some social contact and support.

Can you and your brother hire caregivers so that you can have more free and quality time to spend with her? Time to just enjoy one another and not doing chores, errands, etc.

A gerontologist and/or social worker might be helpful to her.

I think she's lost her sense of identity and usefulness as well as losing your dad.