T&P's mom, that's a tough one!! The drawbacks to these kinds of things is there can be so many reasons that could have started this behavior.
I would probably bring both of them to the vet for a check-up just to be sure and to rule out anything medical. I do think poor Peanut is stressed which is causing her to pull out her belly fur. One of my friend's cats did this when she was stressed.
The behavior of Tubby attacking Peanut sounds like a similar scenario I had between Scooter and Sam (a siamese cat I had in college). Sam would just attack Scooter for no reason, and it really stressed him out.
Has anything changed in your household at all (anything...no matter how small)?? Is Tubby trying to express his 'Top cat' status over Peanut? It almost does seem to me that he is displacing his frustrations on Peanut for some reason. Is there something frustrating him?
Depending on what the vet says medically, I would try to desensitize Tubby from Peanut. Using a squirt bottle is a good idea to get him to stop unwanted behaviors. Maybe keep the two separated for awhile, and only have them together under supervision. Reward Tubby for good behaviors (ie leaving Peanut alone), so he will associate the pleasant things with Peanut being in the room.
I do get the feeling that something is frustrating Tubby, and Peanut is his target for letting out these frustrations. If possible, find out what is bothering him and elimate the problem. Are there neighborhood cats that wander in the yard? Is there a visiting squirrel that he can't get at?? Observe him and let us know what happens!! Very curious indeed!
**hugs**