Since your niece has a history of mental illness, this is not just a case of tough love. Mental illness is an illness, not a personality problem. My guess is that your niece is off her medications which is not uncommon with people with bipolar, ect.
Unfortunately, when the government deinstitutionalized decades ago, they went too far. It is very difficult to get someone mental health treatment if they are not willing. Most states require that they be a danger to themselves or to others. The problem with that is…if your niece was diagnosed with cancer she would be able to process that she was sick and needed to take certain medication/treatments to get better. However, with mental illness people can not process the fact they are sick and need help.
I have a few words of advice. First, tell her parents to talk to the local department of health and human services. They may be able to step in if the house is unfit. Also depending on the standards in your State they maybe able to help them get her treatment. Regardless, many have ideas and things they can do to help short of involuntary commitment.
Second, they need to stop enabling the behavior. While I said before tough love is not the answer. By intervening they may be preventing her to get to the point where government can intervene and try to get her help.
Thirdly, her family should try to seek out support in an organization like NAMI. Mental illness is very misunderstood. People don’t talk about it—so they are often isolated and have no one to talk to. Friends do not understand what is happening, ect. A group of NAMI they can sit down and talk to people who really understand what it is like to have a loved one suffering from a mental illness.