Death of Cage Mate

Question
I had 5 female rats of various ages living in the same cage; Unfortunatly one of them had been struggling with cancer for a while and i had made the disision to have her put down two days ago to prevent and further suffereing. my question however is that one of my rats that happen to be her littermate/sister seems to be a little distrought lately; she never really payed much attention to the other cage mates and usually stuck near her sister. Im worried that she might be depressed. I dont know what to do; usually if she was the only rat left i would find her a new friend but she already has other cage mates. I've been taking her out and handling her often to comfort her but she doesnt seem to be herself. What should i do? i've been filling her full of treats (which i dont want to continue for much longer considering shes already a bit on the chunky side) and letting her run loose out of her cage. What else can i do to make her happy? Also a little bit off topic but if theres any one who lives on Vancouver Island in British Columbia, Canada; I'd like to recommend Dr. Chris Collis in Victoria BC as he was very helpful and knowledgable during the entire ordeal.

Answer
I am very sorry for your loss.  As for the rat your concerned about, there is really not much else that can be done to be honest. The vet can prescribe a low dose of liquid valium. Rats do very well on it and it can help with depression too.   Also, thanks for recommending Dr. Collins. Its always good to hear it from someone that has first hand experience with a vet rather than simply getting their name from the phone book!