Your question about a close relative caregiver who needs help and doesn't know where to get it has a lot of parts. First, let's consider the mom who has dementia.
You describe the mom as needing constant attention and the daughter as someone suffering from Parkinson's. It does not seem that the caregiver daughter will be able to maintain being a caregiver as she is for much longer. Her own well being is at stake.
First, the daughter needs to consult with a social service agency in mom's area to find out what programs are available for her mom. Adult day services may be an option. Some can only help low income persons, and others serve a variety of participants. These programs can be lifesavers for the caregivers. Other programs can fund in home support services, paid for through local funding. It all depends on where mom lives. Now is the time to find out every possible source of help and go after it. Her Area Agency on Aging is a place to start.
As for the brother, he needs medical attention if his anxiety disorder is not properly medicated or if his situation has changed since any prior prescription was provided. Anxiety is treatable. Other programs are available for blind persons, depending on his area. Again, a local social services agency should be able to direct you to resources and programs for him. One must first ask, and the brother must be willing to participate in any local program or service.
If he is not, his sister need not feel guilty if he doesn't get help. It's his choice. She needs to try her best and recognize her own limitations. She must not continue the habit of "constant phone contact". It's energy-sapping, risky for her own mental health and tends to foster unhealthy co-dependency between herself and her relatives who need help.
Finally, your relative needs to get some emotional support for herself. Her local mental health association, county's professional psychological organization, social services agency or faith based organizations can offer guidance and counseling. She is in a sort of trap that will only be more burdensome with time and more risky to her own health. She needs to learn her way out of it.
There are proven physical health risks to caregivers in her situation. Heart attacks and depression are among those risks. She needs to take action to find some relief or her mother and brother might outlive her.
Many kind, interested and helping professionals are there to assist people in her situation if she reaches out. You, as an observer, can only make suggestions. You can't make her get help for herself and her family members. What you can do is urge her to find all available sources to help her do better than she is doing. I sincerely hope she listens to you.