Hi, I'm Dr STeve. I am terribly sorry to hear about your loss. First and foremost, you are not a cat owner with bad judgement, just a bad situation. I commend you for taking such a concern for your other cat's well-being.
I agree that there is a mixed bag of answers to these questions and I can not change that. I hopefully can help you understand why there is a mixed bag and give you some tips to "try" and see if you can make your cat less depressed.
Some cats tolerate the loss of a companion much better than others. You are right that you have changed your cats world. It is hard to know how hard this will be on her. THe only thing to do is to wait, observe and react to her actions. My personal thoughts are to change as little as possible in her routine. SHe has to deal with the loss of a friend, so if she is doing relatively ok ( not having urination or bms outside the box, not constantly meowing, not having seriously reduced appetite or severe lethargy) , then just letting her go through the grieving process may be the best thing you can do for her.
Removing all things that remind her of the other cat could cause enough extra change and stress to cause her to feel more "lost" As time goes on, the scent of her sister will gradually subside and this is a good "weaning" process. Getting another kitten this soon , in my opinion is not a good idea. You can never replace the cat you lost, in her mind or yours and another kitten will just cause additional change and stress that she will have to cope with. A new kitten down the road ( possibly several months) may be ok, but not this soon and not for the purpose f replacing the lost one.
Cats, again , in my opinion do mourn, but differently than people. They miss their companion and the day to day routines that they have become accustomed to , but they don't reflect and feel sad about past experiences and don't look ahead at what they are missing out on without their companion.
They can and do become mopey and lethargic, but more for the changes htey are experiencing now, not in the past or future. I think you are on the right path with giving her extra attention. This doesn't replace her sister and their activities, but it does let her know there is some other "good things" that can still happen and shoudl perk her up and give her something to look forward to each day.
If she begins to have accidents, then I often will prescribe an antianxiety medicine called clomicalm or reconcile to help relieve stress and anxiety. I usually have clients use this fr 1-2 months to help a cat get through the distress. I only use this if there are severe behavioral problems like frequent accidents or total refusal to eat. If these things are not going on , then I don't use any medicines at all. I hope this has helped. If it has would you please rate my answer. If you have more questions , please continue. '
Thank you for your time and insight, your recommendations make sense and I will follow your expert advice and thank you for your kind words and support in regards XXXXX XXXXX about my cats death.