[SWHouston]

First of all, you're not doing anything wrong...

Dogs are much like people. Some have "stronger" personalities, and others more passive. My Male Puppy Ichabod is much like Bella, giving My (alpha) Male Sneakers a lot of respect, while he plays and nips at My Female Becca constantly. I scold him while I pet Becca, but this barely moderates his activities. Only the animal who is being "bothered" by his actions, can deal with that. So, IF you support Sadie more, she may realize that she can do something about it, and put Bella in her place. IF you see Sadie snap/bite back at Bella, I suggest that you don't intervene, other than petting Sadie to support her actions.

However...
All in all, there's not a lot you can actually do about this. Bella is demonstrating her desire for an Alpha position in the pack, and as I said, it's up to the residents (not you by the way) do defend/or permit this to happen.

I will add...
Your Breeder should have been able to tell you about Bella's demeanor before you got her. Those aggressive/alpha tendencies show up almost immediately when the Pups start playing with each other, and rarely an issue which is considered, when adding a member to one's Pack.

This is a "natural" thing, not a "bad" thing.