No 2 dogs are the same, even siblings, much the same as human siblings. With dog siblings (or any 2 dogs of similar ages in the same home) one is often more confident than the other even though they've been raised together and in the same way, same socialisation etc.
Don't push Bailey out of her comfort zone. A growl is a warning that she's already very uncomfortable with the situation she's in and a bite could be the next step if her warning growls are ignored.
If this is happening out on walks or anywhere away from home, make sure that strangers aren't allowed to approach her and that she's on lead when people are nearby for her safety as well as that of others. You don't want to end up being accused of having a dangerous dog because she's frightened someone. Don't be afraid to explain (firmly, because inevitably there will be an idiot who thinks they know better) that Bailey is nervous of people she doesn't know so please don't approach her. You need to be her champion
If it happens when you have visitors at home you could try the following if the visitors are adults and dog savvy. It's what I do with my two because Louis is nervous of people he doesn't know and barks and growls if put under pressure, while William likes everyone -
When the visitors ring the doorbell put both dogs behind a baby gate or in a room where they can't say hello to the visitors straight away.
Once the visitors are settled, and if they're happy to meet the dogs, let the dogs in. Before you do so explain that they should totally ignore Bailey - no eye contact, no words of encouragement, even if Bailey approaches them.
Don't put any pressure on Bailey. If she chooses not to come in the room but to remain outside of it that's OK. She needs to do whatever she finds comfortable. If she comes in and hides behind the sofa or under a table and watch what's going on, that's fine too.
If you think she's getting anxious or if she starts barking or growling, calmly encourage her into another room or behind a baby gate. Don't tell her off for being afraid.
I've done this with Louis and he's become braver. Dog savvy visitors are allowed to throw a treat towards him and say hello if he approaches. But I wouldn't have done that when the problem was at it's worst. I don't really know what he's like with children because none come to our house, but I would be more cautious than with adults, just in case a child made a sudden movement or noise and startled him.