Losing a companion is never easy for a pet. Nor is losing there play partner. I believe pets mourn just like we do, the only thing is they don't have a voice and cannot talk about there feelings.
Losing a pet of my own 16 months ago and having to deal with two others was hard. Not only was I mourning but they were mourning too. What I did was played with them more no matter how hard it was for me. Interacted with both of them and gave them so leeway because I know they were in pain to. I would cuddle with them and give them treats. I put away our babies toys for awhile because his scent was on the toys and washed all of their blankets. I focused only on them and within a few weeks, our other two were back to normal.
Although the routine changed a little, I did not want to lose the other two. I as a human would have been devastated. Our baby is missed and not forgotten but the other two are just as normal as they were before his passing.