M.P.
I know how frustrating it is to know someone needs mental health help and they refuse. Since you only say that she is not an immediate danger and her husband is also in denial, I suggest that your only recourse is to back off and let them live their lives as they wish to live them. They're adults.
I suggest that you continue to love them, express your concern in gentle ways and not try to push them into something they do not see a need for.
If she's doing things that upset you and/or your family then you'll need to find a way to take care of yourself and family so that you're able to manage your own feelings towards her. This may mean spending less or no time with her. That in it's self might open the door to more discussion about the situation and ways to help. But it most likely will not. Doing so means that you are respecting their wishes while taking care of yourself.
My mother was very unhappy and seriously depressed most of her life. She refused mental health care. I kept in mind that she was from a generation in which mental illness was a stigma as well as there being very little effective treatment for it. I offered to help her find help. She refused. I just continued to love her. Spent what time I could with her letting her know that I cared about her and her happiness. A couple of times over the years she did take antidepressant medication but then a life event would throw her a curve and she'd stop. All I could do was keep loving her. Because of her negative view of life, life dealt her many difficult times. I just had to accept that she could not change.
I suggest that since your mother has been this way for years that you cannot change her nor get her to accept treatment. People really do become less flexible and less likely to make changes even when given help. Mental illness becomes so entrenched that even if we are able to get the person to a professional person they cannot become healthy.
It is difficult and sad to see a loved one suffer but it is their choice. We have to take care of ourselves and let go of trying to take care of them.