Hi Alicia! Welcome to BB!
Here’s the deal. Baby bunnies love everyone. They form bonds with everyone. The catch? They do not form LASTING bonds. Once babies become sexually mature, any bonds they had with cage mates can go right out the window. It doesn’t matter if they are male, female, siblings, not siblings, been together from birth, or introduced later. Hormones can do strange things to bunnies and even the “best of friends” may start fighting once those hormones kick in. This can literally happen overnight too, like someone flipping on a switch.
The best advice for bonding bunnies is to have them spayed/neutered, wait a month for hormones to settle, then start the bonding process. You would do this whether they have known each other since birth or they are two adults meeting each other later in life.
If you really have your heart set on the bunny from the breeder, then get her. Just be prepared to have her set up in a separate cage until they are ready to start the bonding process. There are no guarantees, of course. But this is true whether they were together as babies or introduced later as adults. Some people prefer to wait until their current bun is spayed/neutered and then take them to meet potential friends at local shelter or rescue. This provides the best chance for bonding later on, but it isn’t 100%. 