Adjustment is not easy, it's sort of two fold, it's sad for us and sad for the other fur babies.
As we watch our other two cats go through the day, still looking and smelling where their brother would lay during the day, and not finding him is sad just to see the look in their face.
In the three years that we have had Sweetie Pie, she has never tried or had any desire to go outside, even when the door is left open.
The other day while unloading groceries with the door open, she ventured out on the front porch. I know she was looking for Boo. Luckily I was able to scare her back in the house before she ventured too far.
Boo's blood sister is doing the same thing. When ever the door is opened she sticks her head out just looking to see if she can see her brother.
I know it's just a matter of time, how long, no one knows
Ron