Perhaps the 'her' is a family member. The brother and sister might be watching her in the pool because she's learning to swim and they are there to support and keep an eye on her. It could also be that she's doing something interesting in the pool like trying out a new diving technique, and they are curious to see how it goes.
It could be a story about siblings observing another girl in the pool. Maybe she is a friend, and they are just watching her swim or have fun in the pool.
The story might involve the brother and sister being at the poolside and watching 'her' for a particular reason. It could be that she has a special talent in the pool, like being able to hold her breath for a long time or swim really fast. They could be in awe of her skills and are just observing her every move in the pool.
She could be their neighbor. Maybe they are all at a community pool and the brother and sister are just casually watching their neighbor swim.
Once upon a time, there were a brother and a sister. They went to the pool on a sunny day. The sister was excited to swim and the brother was more interested in just sitting by the pool and reading a book. The sister jumped into the water and started splashing around, trying to get her brother to join her. Eventually, the brother put down his book and dipped his feet in the pool, and they had a great time just chatting and enjoying the sunshine by the pool.
The phrase 'glory hole' is rather inappropriate, so if we assume it's a made - up or misnamed place in a story. Maybe it could be a story where the sister is in a special place (not a 'glory hole' in the inappropriate sense) and the brother is watching her. For example, the sister is in an attic full of old treasures and the brother is watching her explore with a sense of curiosity and protectiveness.
BFF stands for 'Best Friends Forever'. In this context, it refers to the sister's best friend in the story.
At the pool party, the brother and sister had planned various activities. The sister made some delicious lemonade for everyone. The brother set up a floating obstacle course in the pool. Their friends were eager to try it. The sister went first to show how it was done. She had a bit of trouble but managed to complete it with everyone cheering her on. Then the brother and his friends took turns, and it was a day filled with joy, laughter and lots of pool - related fun.
It could be about a sister who encourages her brother to go to school every day. Maybe the brother is a bit lazy or has some problems at school, like being bullied. And the sister gives him the courage and support to keep going to school.
One activity they can do is play water volleyball. They can set up a net across the pool and hit a beach ball back and forth. It's a fun and active game that requires teamwork and can also keep them cool in the water.
In a family, there was a sister, Anna, and her younger brother, Ben. Anna was very interested in different styles of clothing. One day, she thought it would be interesting to crossdress Ben as a girl. She started by choosing a flowery dress for him. Then she used some of her old jewelry to accessorize. Ben was a bit shy about it at first. But as Anna started to tell him how they could play a new game with him in this new look, he got more excited. Anna added some light makeup on his face. They then spent the whole afternoon playing games with Ben in his new 'girl' look, creating a very unique and fun memory for both of them.
A sister, Sarah, and her brother, Max, were at home during a long vacation. Sarah was bored and came up with the idea of dressing Max as a girl. She rummaged through her closet and found an old party dress. Max protested a bit but then let Sarah do her thing. She added some of her mother's old jewelry to the look. As she was finishing, their parents walked in and were initially shocked but then burst out laughing. They all ended up having a great time talking about how Max looked like a proper little girl for a while.
There could be a story where a little sister in the pool discovers the importance of respecting others' space. She might be very excited and splashing around a lot, not realizing that she is getting water in other people's eyes and making them uncomfortable. When someone kindly tells her, she realizes her mistake and becomes more careful. This shows how we should be aware of how our actions can affect those around us, even in a fun place like a pool.