Have you tried a feliway diffuser? It's worth a try.

I moved in with my mother last September to be her caregiver.  My two
cats, Stella and Rufus, came with me.  Rufus gets along with
everybody, but Stella and my mother's cat, Sweetie don't get along at
all.  Sweetie has always been affectionate with me, but she was
definitely my mom's cat.  Now Mama has passed away and Sweetie and
Stella are at it more than ever.  I really feel obligated to keep
Sweetie.  She's about 7 and is unlikely to be adopted.  Do you think
this animosity will eventually pass or am I doomed to a future filled
with "cat fights"?  Spraying behavior has increased as well.  I even
found some dried urine on the kitchen counter!

Thanks,

Paula

How long have you all really been together? I inherited my brothers cat and
here we are 15 months later and she still has trouble getting along, and
yet, she also can sometimes actually play with them. I do, definitely, have
to keep Feliway diffusers going. I do timeout when it's obvious there is too
much nitpicking on each other going on. Overall, I think in a year or so,
they will all be OK, but for her it is just taking longer than I've had
before with a cat. I think I would try starting over with a long intro
period if it is too bad with Sweetie. Also add the Feliway. It can really
help.

Karen