How to tame my human aggressive Silky Terrier?

I have a 3 year old Silky Terrier that up until last year was as sweet as can be. I lived with my ex for a while and I strongly believe that his sister corrupted him. At first he was only aggressive towards his sister; but then he started to become more and more aggressive towards not only me, but my ex as well. Over the past six months he has bitten me and drawn blood four times. I've noticed when he knows he's done something wrong, or thinks he's going to get into trouble, he becomes irate. I try to be calm and cool to try to keep him that way, but it doesn't work. When he's upset, there isn't any calming him down until I can get him on to his side and calm him down that way. He's socialized with other dogs and humans, but for whatever reason he will be quick to flip on the nearest person if they do something he doesn't like. When training him, he doesn't like when I offer him a treat, he snaps at me. I can't put treats in his kong because he becomes possessive and aggressive. I'm not sure what to do with him, I don't want to give him up, but I also don't want to pay a ton of money to try to fix a dog that may be unfixable. How can I become the pack leader to him?