There are various possibilities, but the most likely explanation for this sort of story of an older person suddenly behaving inappropriately, would be dementia and/or a small stroke --- causes of brain damage, especially involving the frontal lobes of the brain, which lead to the person losing their good sense and good behaviour, losing their inhibitions, and behaving in embarrassing ways.
What is needed, fairly urgently, is to devise a way to have him seen and assessed by a good local psychiatrist, and if he too strongly resists that, a neurologist, for a proper and detailed assessment.
Treatment and management of the problem will depend on the corect diagnosis, but some behavioural methods supervised by a psychologist and some medications supervised by a psychiatrist can reduce the bad behaviour and make it easier for him to control himself and for him to be guided towards behaving better, by his family.
It sounds from what you say as though there is also a possibility that for some peculiar reason your sister-in-law may be leading him on and encouraging this ; maybe she enjoys the extra attention or has a malicious interest in stirring up trouble within the family, but your mother and yourself should perhaps discuss this with your brother, so he can talk to her and make sure she stops encouraging this, and starts discouraging it.
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