By including a sister character, we’ll likely see how a traditional Chinese child is expected to act, quite differently than Mulan does, and how both the sisters have grown up over time.
We know that the new Mulan will open with a scene with the lead character, and one assumes, her sister, as children.
Their differences won’t make them adversaries in any way, Jason Reed says the two will still be close, but also quite different.
This scene was added in order to show that Mulan’s unique character is something that has been part of her for years.