Here's the thing. It's not that she won't correct the behavior. It's that she can't correct the behavior. Because this behavior is fear based, and you can't whup fear out of a dog. It just doesn't work that way.
If the dogs' behavior has escalated to them biting you numerous times, then it's probably time to call in a qualified animal behaviorist. Normal sane dogs don't go out of their way to bite. Biting normally only happens after a series of previous warnings has been ignored. Yeah, your niece might have one dog with a loose screw and a bitey temperament. Four bonkers bitey dogs? Unlikely. That's too much of a coincidence. Something is wrong with the household dynamic, and it's pushing these animals into being jerks. You need someone who can look at the whole situation and figure out what's at the root of it.
What I would do in the meantime is partition off sections of the house to set up safe zones for the people and pets. I have baby gates across my kitchen doors and my bedroom doors. This makes it possible to lock the dogs into or out of the kitchen and bedrooms. I originally got the gates to help with housebreaking and to give my cats a safe space. Now I use the gates daily: when the dogs are rioting and need to be separated to chill out, when I don't want muddy wet feet on my bed, when I don't want the dogs bothering a guest or repair guy, when I don't want to have the dogs underfoot as I'm cooking, etc.
As far as the treats go, food is not always the way to a dog's heart. It's like if a cop rang your doorbell and handed you one of those edible fruit and chocolate bouquets. Yeah, you might like fruit and chocolate. You might love getting a fruit bouquet from your sweetie. Cops delivering fruit and chocolate, though? No, that's deeply deeply weird. It's not going to make you feel comfortable, and it might actually leave you feeling worried. (Why are they doing this? Why am I singled out? This is so bizarre. What's really going on here?)
If you do want to continue trying to work with the dogs using treats, there's a technique to it. Flick the treat past the dog, so the dog needs to move away from you to get it. This gives the dog two things it wants: more distance from you plus the treat itself. Then move away from the dog without seeking further interaction. Be like Santa Claus: make your delivery and then vamoose. If the dogs are interested in interacting, they will sneak over to check you out like little kids getting out of bed on Christmas Eve. If they aren't, just continue on about your business.