Multi-cat question

Because your kitties have each other you probably won't have much trouble. My experience is below. Hope it helps.

When my dog passed away my kitty Knug was so depressed. He barely ate, stopped playing, did not use the litter box, stopped seeking attention and snuggling, and doubled the amount he slept.

After giving him a couple weeks to grieve he was not improving and started losing weight. After eliminating any other health issues the vet and I decided to put him on buspar. Basically, I had to give him prozac to recover. I did not have any other animals at the time. He was on buspar for six months and it really made a difference and got him back to normal.

I met and married my husband and his kitty, Penalope, and was back to two animals in the house. Penalope passed away in September after 21 years with my husband. Knug again was depressed but not as severely. A few Feliway plugins and lots of extra attention was all that he needed to recover.

Two months after losing Penalope we brought home Damage. She perked him up big time! In March, six months to the day after we lost Penalope my precious Knug passed away unexpectedly at the age of 16. Damage was only six months old and just beside herself. She spent a lot of time wandering the house and crying. We could not take the sad wail of her looking for him. Even though it was a bit soon for me we went to the shelter and found my gorgeous Swiffer who is two months younger than Damage.

As soon as Swiffer was released from quarantine it was instant buddies and Damage was back to her normal chatty self. The sad wailing and wandering about the house ceased.

First, I am sorry for your loss. Cats do grieve when they lose a best friend or sibling, and hteir behaviour can change. This can happen even in a multicat household. My Wellington grieved for 6 weeks after losing his brother, although I had four other cats at the time. He was depressed, cried all the time and went round and round the house looking for Napoleon. Things improved when Biscotte joined us three months later - he instantly adopted her as his best friend and they are still best buddies five years later.
All you can do is comfort the grieving cat, talk to her, give her extra attention and divert her in any way you can. Every cat will be different in the way they handle a bereavement.