Nursing home neglect/abuse

My sister was living in a mental health type group home. Because she had suffered a stroke in her thirties she had only limited movement one side but could still walk. She fell at the home but was not attended to. When I found out I was called to speak to the Residence Director and was at first ignored but then told that they were placing her in a nursing home (I'm assuming as punishment for complaining). After leaving her friends of many years in the home, she was admitted to a nursing home where of course she died within the month. She was only 53 and one of the youngest residents there. The autopsy said she had a heart attack. The nursing home said she had fallen. She seemed okay when I visited two days earlier and when I spoke to her the night before. I feel lost and don't know what to do. It's been six months but I've been dealing with the lost of two other close family members deaths six months apart and couldn't handle this one until now. I did obtain the autopsy and wondered if they also failed to treat a past fall or what could have caused this to happened. This home has been sited in the past for negligence. How can I request that this place be investigated?