Introducing new cat after loss of litter-mate

Question
Hi, I had two tortoiseshell cats that are 7.5 years old, sisters. One of them recently died leaving me with Gertie, who has always been very loving and sweet. I have rescued what I think is a Ragdoll/Norwegian Forest cat mix (from what I've read) female estimated at 3 years old about 6 weeks after our loss. I kept Rose (new kitty) completely isolated for 10 days and have tried to slowly introduce them since. Feeding on each side of the door without line of sight, peeking through cracked doors, etc. It's now been 6 weeks since I got Rose. I have double stacked baby gates up where I still keep Rose isolated when I'm not home but they can see and smell each other through the gate. I have recently started letting Rose out while I am home, and she is very loving, calm, and sweet. Gertie is extremely aggressive towards her, walking up to her and hissing, growling, and swatting. She seems to portray the commonly referenced possessiveness and aggressiveness of a tortie, and seems to refuse to accept Rose into the home - especially at bedtime! (Gertie sleeps with me and will not allow Rose in the bedroom). I have noticed that she lays in a position near me that appears that she is blocking Rose from me both in the mornings getting dressed for work and evenings at bedtime. (even when Rose is in her room). I am at my wits end! I have read about and tried reintroducing them, but after 6 weeks I am concerned that Gertie will never accept her. I have also tried Feliway but it didn't seem to help, either.
Any thoughts on what else I can try to help Gertie accept our new addition? Sometimes I think things will work out as she will come sniff her, growl slightly but walk away, and other times, she will see her and stalk over and start beating her up. (swatting and hissing mostly - a couple times she has attacked from the back trying to bite her neck area, fortunately unsuccessfully)
Any advice would be greatly appreciated! Thanks so much for giving this opportunity to seek advice and taking the time for it!

Answer
This is very common behavior for any cat to go through and I won't lie that Gertie may never accept the new cat.

What I always have good luck with is my "food mediator" process. The goal of this is to basically let Gertie know that the new cat is not a threat and life in the house can be normal.

What works best is a large area where there are no objects that the cats can get hurt on when it comes time for introduction as you can expect Gertie to freak out and who knows what will happen.

What you're going to do is get dishes for both cats, as far as possible away from each other and filled with their favorite food/treat (I tend to use either tuna or canned food.) Start with Gertie first as she is the instigator. Get her settled and eating then bring in the new cat in the opposite side of the room, preferable so Gertie does not get interupted, and do the same with her. Once both cats are eating peacefully, gradually begin to move the dishes closer together until they are eating side by side or right out of each other's dish. NOTE: This will not happen right away, so be patient and block out a lot of time for this to work.

Let me know how this goes, but I have yet to have this not work.