Playing basketball can be really fun. The brothers can show the sister some cool moves and she can try to copy them. They can have a mini - tournament in their driveway, just for fun.
Another activity is to have a movie night. The brothers and sister can choose a movie they all like, make some popcorn, and snuggle up on the couch. During the movie, they can share their thoughts and laugh at the funny parts together.
They could go cycling together. The big brothers can help the sister if she has trouble with her bike, like fixing a flat tire or adjusting the seat. It's a great way to explore the neighborhood or a nearby park.