Seeking expert counseling is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
It is true that your brother-in-law abused your sister emotionally (at least) for years by his behavior and she has become quite traumatized by it. It is possible that he did
bring women into the house while she slept, in the past, if the house layout permitted it. Considering his age it is unlikely that he has been cheating on her lately, but it is not impossible and he does have a history, which is no doubt deeply burnt into her consciousness.
She may even be suffering from a form of PTSD, an anxiety
disorder, that can emerge years later because of past traumas, rather than dementia or schizophrenia
She appears rational and normal to you.
You say that you do not want to get involved, but you are her sister and have been with her longer than her husband and children. The husband has been the offending party and may be using his influence to turn the children against their mother, possibly to get her out of the home.
I believe that you should follow your instincts, support your sister, give her in-person support as much as possible, and explain to her about PTSD which is not a psychosis
, but an anxiety response to real trauma from the past, perpetrated against her by her husband.
She may have dementia or delusions, but give her the benefit of the doubt, as your own eyes tell you otherwise. I am sure that you want to protect her from being railroaded into a mental health or nursing home facility, pumped up with drugs and left to live out her years in a drug-induced stupor, which could be the outcome of this in a worst-case scenario.
I applaud you for your support.
Elliott Sewell, LPCC, NCC, CCMHC