My grandmother had told me this story about a friend of hers. Let's call her Kalyani.
This happened more than 70–75 years ago.
Kalyani couldn't have been more than 16 or 17 years old when her parents married her off to a guy (say Shekhar). It was very common during those days to get married at an early age and not be bummed about it.
So Kalyani happily took her leave from her paternal home and went to her husband's house.
The first night, however, came as a shocker to her. She kept waiting for Shekhar in their room and eventually dozed off.
Some time past midnight, Kalyani was woken up by someone lying down next to her. To her horror, it was not shekhar. It was his elder brother.
Naturally, Kalyani starting shouting and asked him to leave. All the family members came into her room.
When she told them what had just happened, everyone seemed unperturbed. The elder brother’s wife (who should ideally have been devastated) asked her to stop the drama and adjust.
It took Kalyani a while to understand what really was going on in the family.
So Shekhar and his sister-in-law (elder brother's wife) were having an affair. Everyone in the family knew about it. Even the elder brother had somehow accepted it very happily.
The family had not expected that a young woman from that era would have the courage to speak up for herself. Like most women from that time, she was expected to compromise with her situation and move on with life.
Kalyani, clearly, was different. She stood for herself. She left the house and went back to her parents' house.
They got her educated and she became a teacher. Years later, she married a wonderful man who respected and loved her.
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Edit: I understand that people are finding it difficult to believe the story. If you don't, please move on with quora surfing. Treat it as a fake story. It's perfectly fine. And if you believe that customs like dadhicha pratha, draupadi pratha, paro pratha etc are not fictional crimes, then I don't see why it's so difficult for you to believe this incident.