A great activity for a brother and a sister at the pool is to create their own mini - water park. They can use pool noodles to make slides and ramps, and use buckets to pour water and create waterfalls. It's a creative and imaginative way to have fun at the pool.
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.
They could have a treasure hunt in the pool. The sister or brother can hide some small waterproof toys or trinkets in the pool beforehand. Then they both search for them. It adds an element of mystery and excitement to their pool time.