Help- my sister-in-law is taking advantage of my elderly in-laws. Any suggestions?

She blackmails them into paying her rent in order to be able to see their grandchildren! My in-laws are in their mid-80’s but have paid their adult daughter & son-in-law's rent for the past 13 years. My brother-in-law has never been able to keep a job, he doesn't even try to find one anymore. My in-laws have helped with their schooling expense, even paid for loans they co-signed on, in addition to giving them numerous vehicles over the years. Two years ago my mother-in-law estimated they had gave them close to $150k.
Now my in-laws are not buying items they want/need because of the cost of supporting another family. They have had numerous health issues lately and I hate seeing them going without items they need! When we suggested that the sister-in-law applies for Welfare herself she plays a guilt trip on her parents by saying "You don't want your grandchildren to live in the projects do you?" When my husband & I have tried to offer suggestions for stopping this dysfunctional cycle- my mother-in-law warns us that her daughter has quite a temper and that we should not make her mad! This leads us to wonder if she is making physical threats besides financial threat. My husband & I live 6 hours away in another state. My in-laws don't seem to realize that there are government programs that will provide housing for their daughter so they do not have to support them financially. We are in need of a 3rd party to council my in-laws , someone in the state of Illinois who will be able to assist my in-laws with breaking their financial responsibility and find their daughter & her family a safe home. Suggestions for government agencies that will speak to BOTH my in-laws and sister-in-law like a family mediator or some type of financial planning assistance for the elderly.