The bride might have chosen friends over family. Mom expected family loyalty in the wedding party selection. So, she's upset that the sister was overlooked.
It could be that the sister was really looking forward to being a bridesmaid. Mom saw her disappointment and got upset on her behalf. Maybe the sister had been helping a lot with the wedding preparations, and not being made a bridesmaid felt like a slap in the face to Mom.
There could be some past family issues at play. For instance, if there were family feuds in the past and the bride not choosing the sister as a bridesmaid could be seen as a continuation of that rift. Mom, being more aware of the family history, is upset as she hoped the wedding would be a chance to heal old wounds and bring the family closer together through the sister being part of the wedding party.
One possible reason could be that Mom had some traditional expectations. Maybe in her mind, it was a family honor for the sister to be a bridesmaid. Another aspect could be that she was looking forward to seeing a certain family dynamic in the wedding party, and without the sister as a bridesmaid, it disrupted that vision. Also, it might be that Mom thought the sister was really close to the bride and was shocked when she wasn't chosen.
It could be that he failed at something important like a difficult task or a competition within the story of the fanfic. This would surely make him feel down.
Maybe the fanfiction portrays him in a negative light, like making him out to be a bad person or having him do things that are completely against his real - life values.
One reason could be for a unique and inclusive family photo. It shows that traditional gender roles don't have to be so strict. Maybe the family wants to break the mold and create a memory that's different from the norm.
In many stories, disrespect is also a cause. Saying rude things to mom or not listening when she's talking. Moms spank to teach kids to respect others, especially family members. It's their way of instilling good values in a rather tough but sometimes necessary way.
One reason could be a lack of female candidates. If the bride doesn't have enough female friends or relatives to be bridesmaids, she might turn to a boy in the family or among her friends. Another reason could be for the sake of novelty or to make a statement. Some people like to break traditional gender norms in a wedding for the shock value or to show how modern and open - minded they are.
Being disrespectful is another reason. If you talk back rudely to your sister or don't listen to your mom, you're likely to get punished. Say you roll your eyes at your sister when she asks you to do something simple like pass the salt, that's disrespectful. Your mom might ground you and your sister might stop sharing her things with you.
One common reason is disobeying house rules. For example, not cleaning your room or staying up too late.
She might have been curious to see how you would look like as a girl. It could also be a part of some kind of game or bet she had with her friends. Another reason could be that she was trying to create a funny memory or just wanted to tease you in a light - hearted way.
One possible story could be that a family already has a son. Then they adopt a daughter, which is like the brother getting a sister. Shortly after, the mom discovers she is pregnant again, perhaps unexpectedly, bringing more joy and changes to the family.