Often, they end happily. The couple overcomes the initial obstacle of the brother's reaction. For example, in many stories, the brother is initially angry or shocked but then comes around and accepts the relationship. So, the two lovers end up together, happily in love.
These stories can end in various ways. Sometimes, the girl realizes that her feelings for her brother's best friend were just a passing phase and they go back to being friends. But more commonly, if it's a true romance story, they will end with the couple together. They might have to face challenges like family disapproval or misunderstandings, but they fight through them. For instance, the brother might be worried about his friend breaking his sister's heart. But once he sees how much they love each other, he gives his blessing and they live happily ever after.
Often, they end badly. Either the friendship is completely ruined because of the betrayal. Or, if there was a romantic aspect, the relationship doesn't work out and they end up losing both the friendship and the romantic connection.
Some end happily. The two friends realize their mutual feelings and start a romantic relationship. They build on the strong foundation of their friendship and often have a very deep and loving connection.
Typically, it starts with some sort of innocent encounter. Maybe they share a moment while the brother is around but not really paying attention. Then there's often a period of secrecy. They don't want the brother to find out at first because they know it could cause problems.
Often, they end happily. Since they started as best friends, they already have a strong bond, and in the end, they usually live happily ever after, perhaps with a growing family or achieving their dreams together.
Often, these stories can end badly. There could be jealousy issues that arise. For example, the bi guy might start to feel left out if his wife and best friend become too close without him. Or the wife could develop stronger feelings for the best friend than she intended, which could lead to the breakdown of the relationship between the bi guy and his wife or between the bi guy and his best friend.
Well, sometimes the wife realizes her mistake and tries to end the affair before the husband finds out. But there are always consequences. The relationship between the wife and the husband is strained. The best friend may feel guilty and disappear from their lives. In other cases, if the husband does find out, there could be a big fight, and the end result is a divorce. The social circle also gets affected as people take sides.
However, there are also sad endings. In '5 Centimeters per Second', the main character Takaki Tono has a series of unfulfilled loves. The story ends with a sense of melancholy and longing as he and his childhood love, Akari Shinohara, have drifted apart over time.
Often, they end happily with the main couple getting together. For example, in 'Fruits Basket', Tohru Honda and Kyo Sohma end up in a loving relationship after all the trials and tribulations they've been through.
Often, it starts with a deepening of the friendship bond. For example, they might start sharing more personal secrets than before.
They usually start when they spend a lot of time together. You know, doing things like going on long hikes or cooking together regularly. As they do these things, they start to feel a deeper connection. It's like the friendship deepens to a point where it turns into love. They begin to notice the small details about each other that they previously ignored, like the way the other person smells or their little habits.