Hi guys, since I had read that Belle was originally going to have two sisters in early developments of Beauty and the Beast. But were ultimately cut because the story will be like Cinderella, here's my reason on why and what if they are in the movie instead.

Belle as an Only Child

This is mainly because I kind off feel bad for Belle and her dad, especially the former because she has nobody except for her dad to talk to. Especially if you have no mother or sisterly support.
If they were included in, the filmmakers will definitely tone down their characters to distinguish themselves from Anastasia and Drizella. Although a minor role to help with their father, Maurice.

How They Will Benefit the Story

As mentioned earlier, if their characters are changed and they would definitely help Maurice talk to Gaston regarding the Beast and when Belle goes missing since their father's captive and his release.
They would also be helping when the beast is killed by Gaston, thus making them princess or Grand Duchess upon Belle's marriage to Prince Adam after the transformation scene.

Conclusion

So this is basically my opinion on why I personally felt that the 2 sisters will be in the final film.