Not Friend's Anymore

A year ago, my big cat Coal nearly died from a urinary tract infection. Before he got sick, he and his half sister Munkie got along fine. But after he recovered enough we let them interact they did nothing but growl and hiss at each other. We have to keep them apart other wise they fight. Is there anyway to get them to not be that way to where we can stop dividing the house, and anyway to get them to be nice to a new cat if we get one?

Coal probably smells different after the meds and visits to the vet so Munkie treats him like a stranger. I suggest you read the stickies on introducing cats (which will also help when you get another kitty).

I can't remember which thread, but I was perusing one today that recommended rubbing one cat all over with a towel, and then using the same towel to rub the other cat. The next day, you should do the same with a new towel, starting with the second cat you rubbed before. It will help to accustom them to each other's scent.

I bet it would help if the towels you use have been used by you or someone in your family so that the cats smell like "home" afterward.

Welcome to TCS! I agree with proceeding as a new introduction You could also include laying on the bed with one cat on each side and blocking mutual viewing with pillows. Then stroke them simultaneously. Eventually, you can remove the pillows & let them have a glance at each other while enjoying the petting. Watch their ears & tails for signs of agitation & at the first sign, put up the pillows again, till they relax.
Sending prayers and vibes for returned harmony to your home

Whenever I introduced a new cat, I put them in a spare bedroom with a screen door on it. That way, they can get used to eachother again. I would put a string under the door so they could each have an end to play with. Feed them one on each side of the door. I found that to be a safe, reliable method to use. Hope it helps you to re-introduce these two.