Poor Teddie ~ sounds like she's having a bit of a rough time of it
When you say "love-hate" relationship ~ what bit is the "love" bit as it sounds like they aren't getting on very well & it's getting worse.
As it's now got to the stage that Teddie has developed stress cystitis, I think you may need to consider fairly drastic action, in order to prevent it getting worse & for the sake of Teddie's health.
Is there any way that you can give a Teddie a separate place/room of her own (a "comfort zone") where Cooper can't go? Until Teddie is able to feel safe & relax, & allow the chronic stress she's suffering to abate, then it's likely her health & behaviour will deteriorate. Only by having somewhere safe away from Cooper, where she can sleep properly & relax, look after her own bodily needs & eat & drink, will she start to improve. It may take days, weeks or even a few months, before her behaviour & health start to improve, depending on how long this has been going on.
If you are able to give her a room of her own, it will need to be kitted out with the following:
- A choice of warm, comfortable sleeping & resting places at different heights & in draft free places.
- Food & water
- A scratching post or equivalent
- Toys
- A litter tray, as far from the food/water/beds as possible & to be cleaned regularly
- The company of you & your family, provided that she enjoys this ~ even just spending 5-10 mins playing with her, grooming her, cuddling her, whatever makes her purr, at regular intervals throughout the day.
- If she gets on well with Broxi, then perhaps after a while, you could allow Broxi into Teddie's "comfort zone"?
The reason I'm suggesting going down this route is because, from your description, Teddie is living a life full of fear & anxiety primarily caused by Cooper's behaviour. Not for one moment am I blaming Cooper & there are also things you can do with him to stop him bullying Teddie. But until Teddie starts to realise that she's got a safe haven where she escape to, then the stress/fear/anxiety won't go away & is likely to get worse. We all know what it's like to live in fear & be constantly stressed, vigilant & unable to relax, & we all know the harmful effects that this has on our health ~ it's no different for animals. So you need to remove the source of the stress before Teddie can start to feel better & her health improve.
With Cooper, it's likely that the reason for his "bullying" behaviour towards Teddie, is because of concerns over whatever he perceives to be important resources. So by increasing the important resources, you may make him feel less worried that there's not enough to go round. By resources I mean the things I've suggested putting in Teddie's comfort zone. And lots of them!! So at least one litter tray per cat (& some behaviourists suggest 1 per cat plus 1 extra), several feeding stations, several water bowls (they don't need to be in the same places as the food), several scratching posts/sites, lots of warm snug hidey holes at different heights, preferably off ground level. Also, you & your family may be important resources to your cats, so you need to try to make time for them all & intereact with them in ways they enjoy. If they love their food, try hand feeding tasty morsels, or cuddling & grooming them.
If possible you'll need to prevent any contact between Cooper & Teddie, whilst Teddie is recovering. But if you can't prevent contact, you must try to ensure that any any encounters are neutral & there's no agression, chasing, traping etc.
Once Teddie's starting to feel better, then there are ways to start reintroductions (via counter-conditioning) that may help prevent a return to the previous aggression.
I hope this helps a bit ~ if you have any queries, or want to provide further info, I'm happy to try to help. An alternative is to perhaps contact a feline behaviourist via the APBC who could visit your house, observe your cats, & try to give more practical help based on your personal circumstances.
The love/hate bit, well that's easy. They will sit and groom each other quite happily on numerous occasions, especially Cooper, he is always washing her face. They will lie sleeping together on the sofa etc, and at these times seem the best of friends. 