A little over a year ago, a cat showed up on our doorstep. Since then, we've taken her in and she lives with us. She spends most of her time indoors, but she doesn't have a litter box, as she goes outside to do her business. Since she was most likely feral, we took her to the vet to get her shots and be spayed, but they found that she had actually already been spayed. She's pretty skittish, but will meow at us when she wants to be petted while she eats.
The issue is that when there are other cats outside, she makes some pretty horrendous sounds. The hissing is fine, but other times it sounds like she is dying. In general, she tends to appear as the aggressor, as I haven't seen any of the cats outside act threatening toward her really. What can my family and I do to help her not feel so threatened or scared by cats outside?