There could be a theme of sacrifice as well. In the novel, the characters might have to sacrifice something, like their relationship with family or their reputation, in order to pursue this forbidden love. And perhaps the theme of redemption. If they make mistakes along the way, they might seek redemption either in their relationship or in their own personal growth.
Well, a theme that might be present is the exploration of secrecy. In this kind of relationship, secrecy would likely be a big part. Also, the concept of self - discovery. The protagonist may learn a lot about herself through this forbidden love. Maybe she discovers her own strength in defying the odds and dealing with the complex emotions that come with loving someone she's not supposed to.
Typically, in a story like this, the female lead is central. She is the one experiencing the forbidden love. Her father's best friend is another key character. He might be a complex character, perhaps torn between his friendship with the father and his feelings for the daughter. And the father, he represents the family bond and the traditional values that are being challenged by this relationship.
Friendship is also a big theme. It shows how the relationship with the best friend can be tested when there are romantic feelings for her brother. It might also explore themes of loyalty and betrayal in the context of friendship. For example, if the protagonist starts a relationship with the brother without telling the best friend, it can lead to a lot of drama. Another aspect could be self - discovery. The main character may learn a lot about themselves, their desires, and what they are willing to risk in the process of getting involved with the brother.
One common theme is the realization of a deeper connection over time. For example, as friends, they've shared many experiences and then suddenly, a moment makes them see that there's more than just friendship there.
Another theme is social opposition. Society often stands in the way of the lovers. In 'Wuthering Heights', the social differences between Heathcliff and Catherine make their love difficult. The people around them, including their families, are against their relationship because of class and other factors.
One common theme is sacrifice. In many forbidden love stories, the lovers have to sacrifice something, like their social status or family ties, to be together. For example, in 'Romeo and Juliet', they are willing to risk everything for their love.
A frequent theme is loyalty. The best friend has loyalty to your brother, but there might also be a new loyalty developing towards you in the story. Also, self - discovery is common. As you interact with your brother's best friend, you might learn new things about yourself. For example, you might find out that you're more adventurous or more empathetic than you thought.
One common theme is family. These novels often focus on how the unexpected arrival of a baby changes the relationship between the mother and the father, and how they come together to form a family.
Another theme is the discovery of a new side to someone. The girl in these stories usually only knew the brother's best friend in a certain context before. But when she starts to fall for him, she discovers new aspects of his personality, like his hidden talents or his vulnerable side. This new discovery deepens their connection.
One common theme could be loyalty. They might have stories where they always have each other's backs no matter what. Another theme could be adventure. Maybe they go on hikes or explore new places together. And also, there could be a theme of growth, as they learn from each other and become better people through their friendship.
The theme of sacrifice also frequently appears. The lovers may have to sacrifice a great deal, such as their reputation, family, or even their lives for their love. In 'Atonement', Robbie and Cecilia's relationship is torn apart by false accusations, and they both end up sacrificing a lot in the name of their love, which was misinterpreted at first.