My spouse and I have two cats who we adopted as kittens from the same litter - our two twin girls. They have been best friends since kittens, playing together, snuggling and sleeping together, etc. They are almost 3 years old now. Due to unforseen circumstances, my mom moved in with us last year and left her cat with my sister at the time. My mom just returned from visiting my sister and brought her cat back with her (5 year old, timid girl), as my sister can’t care for her long term.

We have the “new” cat set up in my mom’s room exclusively, but the cats did see each other through the carrier when we first brought her in from airport/car trip home.

One of our two cats immediately hissed and hit the carrier upon realizing a cat was in it. This seems normal from our understanding. We immediately got “new” cat into my Mom’s room with her setup.

The rest of the evening, our girls seemed fairly normal with the one who hissed being a little more skittish than usual but she can be skittish by nature at times. Both of our cats slept in our bed as usual, near each other and snuggled in, and woke up this morning and ate breakfast seemingly fine.

Fast forward to lunchtime today. Our skittish cat saw her sister come around the corner and suddenly became very upset, hissing and growling intensely - which she has never done and certainly never towards her sister! Her sister seemed confused and scared. We gave th both churu and treats near each other and her sister was interested in rubbing up against her again, but the skittish one again hissed, growled, and ran away.

We expected them to need a very slow introduction to the new cat who has remained in my mom’s room, but we are really worried and scared about her reaction to her own sister.

Any advice/experience with something similar? I have never had this happen in the past with cats all my life - slow/scared reactions to the new cat yes, but never seen them turn on familiar cats this way. 😭