After a few weeks of poor Twix being tormented I think we have a solution that will keep everyone happy (for the most part). In order to keep us happy and sleeping with out waking up to growling or snarling we moved a litter box into our bedroom and would keep Twix in with us for the night. That was a fine temporary solution but didn't make it any better for Twix at other times of the day. The poor girl was just terrified all the time!
To make her happy we now have the obstacle each time we use the stairs of a baby gate. Twix gets the upstairs and Ben gets the downstairs. Food and litter are in both locations. You might say that a cat can jump a baby gate and in theory yes they can. Although cats can jump that high they need something to land on at the peak of the jump and they don't hurdle things that large. It has been 5 days and so far that theory holds true.
We had to be strategic with the placement of the gate. We tried the top of the stairs because we didn't think Ben would fit through the rungs of the banister. Reese and Twix can so they could come and go if they wanted but it would keep Ben away from Twix unless she dared to go downstairs. Ben is a determined cat because we watched him squeeze and maneuver himself through the rungs. It wasn't easy for him but he did it. It's a good thing that the stairs are in an 'L' shape so the gate is now at that 'L'. There aren't any rungs for anyone to fit through.
I did have a surprise yesterday though, Twix was downstairs!! Since we don't need the baby gate for the safety of a child we have it pressure mounted. The large post of the banister is decorative and not fully square so the top grip doesn't touch on 1 side. Due to the shape of the gate on the side and the banister there is enough room for Twix to squeeze through. It's no where near large enough for the other 2 to even think about getting through. I guess we'll have to keep an eye on this because I guess Twix may still decide to tempt her fate on occasion :)
I take Reese up with me sometimes because it doesn't really matter where he is. At night and when we are gone I make sure he is downstairs because he has a new favorite spot on top of the kitchen cabinets. I don't allow my cats on the counter but will let that rule go as long as it is just the hop from the counter to the refrigerator to the cabinet.
Twix is more relaxed now, Reese really doesn't care and Ben is probably the most unhappy because he sits at the gate in the morning and meows because he can hear us upstairs. Oh well, he'll get over it. Besides, it's all his doing...if he would just leave her alone we wouldn't have to do this.