What should you do if your sister's husband behaves inappropriately with you, but your mother and sister insist on maintaining a good relationship with him for the sake of your sister's happiness?
You should avoid any inappropriate situation with any one but you don’t need to create a problem in the family. Keep yourself away from any inappropriate situation which might lead to sinful act or sinful thought, but don’t inform others who can create a problem in your family.
Wassalam.