Dog Jealousy and Aggression! Please Help!

I have two smaller breed dogs, one named Bridgett and the other Casper. Casper is a Bichon Frise of 7 years while Bridgett is a mutt, (although suspected of Maltese, Shih Tzu, and Bichon Frise origins) of 6 years. Casper is a very submissive dog who has no aggression whatsoever, but Bridgett has become very jealous of him and food aggressive. When Casper wasn't looking, she would take his treats or food or sometimes snap at him so that she could have it. Casper would never fight back against her, so she got away with it. However, when we would go to take the treats from her, she would growl or snap at us. When he would go to jump in a chair to be with someone in my family, she would growl at him, and when he would make the jump, she would lunge at him with full intent. Bridgett has a problem with one of her back legs, though, so she can't jump properly. Eventually, Casper was too scared to jump, and so my family physically had to put him up on the couch or in a chair. Tonight, Casper was doing nothing except eating his nightly treat, and Bridgett attacked him. Unfortunately, I was unable to see what had happened, but I heard him squealing for a good few moments. This behavior has not gone as long as she was a puppy, although smaller forms of jealousy have, such as when I would pet Casper, she would follow suit and come and join. If I had to guess, it's probably been going on less than a year.. Or similar to one. I have no clue where to start. My family refused to attempt to train her out of this. Since I didn't have control of how she was raised as a puppy, she was not properly trained for most commands. Please help/give advice!! She's a sweet dog when she's not being this way!!