This'll be a bit long, trying to get all the facts out there.
Two cats, both black and white "Felix" types. Both live at my mum's house (large property in countryside with tons of indoor and outdoor space).
The oldest one (male, mine) has been with us about 6-7 years, the younger one (female, my mum's) arrived about 3 years ago. Both were found in the field behind our house when they were kittens.
The younger one won't stop attacking the older one, she won't leave him alone, growls at him when he's on the other side of the window waiting to come in, goes for him if he walks into a room. For some reason, the older one isn't defending himself.
When people are around to see the attacks, we throw water on the younger one (loud noises have no effect) who stops, but then will start again later that day/a few days later.
My cat (older one) has obvious injuries, the biggest (so far) being bad bites on his ears and one ear now has quite a few bald spots.
They will also happily sleep together on my mum's bed/sofa/in the same room. The attacks seem to come out of nowhere.
My cat (older) keeps to himself, is usually asleep somewhere, doesn't seek out the younger one.
There's more than enough room at my mum's. It's a very large house, the part she lives in has 2 bedrooms, 2 lounges, an office, a kitchen, 2 bathrooms, hallways etc and the cats have access to various parts of the rest of the house. The garden is huge and is surrounded by fields, the cats often go off exploring, they're not bound to the property at all.
I have no clue what to do. I don't want it to get worse, I don't want my cat to get even more hurt.
Before anyone asks, I can't take him to my house. He's very much an outdoors cat who doesn't defend himself and my village has a bunch of strays who we can often hear fighting, and my garden is small. He likes being near people and I don't want to leave him home alone all day while I'm at work (I work at the office at my mum's house so am there all day every weekday). Added to this, I rent my house out on Airbnb during the summer months and move into an apartment for 4 months where I won't be able to take him. It makes more sense for him to stay where he is, at my mum's.
A situation that happened this morning and I don't know how best to handle:
They were both asleep on my mum's bed, all fine. My cat came into the office and asked to be let out the window. He sat on the window sill and the younger cat followed, sat there and growled at him through the closed window (very loud wailing growls). What am I supposed to do?
Is there a way to get the younger one to just back off? This has been going on for over a year, on and off. Is it likely to get worse? Any help/advice would be really appreciated!