Who is usually "in charge" in the household? Maybe Jazz is trying to challenge Emma or something now that she's grown? Not really sure what the right course of action would be, but taking away their privileges feels like the right way to go about it. If it's resource guarding, she can't guard something she doesn't have.
Strider used to get a little uppity sometimes with other dogs when they were visiting, or we were house sitting with my aunt's dogs. He plays perfectly and sweetly with them. Even the little poo puppy, he spent 45 mins playing tug of war with her Sunday and kept letting her win. But then he would be chewing on a bone or getting a treat, and if one of the dogs came near him he would roar and lunge. I don't know if it was completely the right thing to do, but I took all his stuff away and made him earn it all back. Kind of like being grounded. Anyway, he lost his bed, his toys, his treats. His dinner went in my pocket and he spent three days on a 4 foot leash tied to my belt. He had to do good stuff to earn his regular meal throughout the day. He wasn't even allowed to pee without me telling him okay. When he finally earned everything back (it took about a week) he was totally better though. Better overall. He stopped being a hormonal jerk and he stopped resource guarding with the other dogs. Maybe being grounded reminded him that "his stuff" was really mine on loan to him? Another nice side affect, is that he now unloads his pee in one trip instead of saving it to mark 20 different trees on our walk. Yay for not marking!
Maybe a similar treatment for Jazz would work? I don't know if in the long term it was a good thing to do, maybe one of the more knowledgeable trainer types on here can help?