Cage mate death, what to do?

Question
In October of 2007 I bought two rats, sisters, who were approximately 7 weeks old at the time.  Last night I lost Charlie, to what I believe was a pituitary tumor (looked up signs and it indicated it was probably that).  She went downhill pretty rabidly, didn't even have time to get her to the vet, which I guess I'm sort of grateful that it wasn't a long drawn out painful struggle for her.  Precious, her sister/cage mate, seems to be looking for her, which is understandable.  I hold her to both comfort me and her, but she's always running back towards their house/cage.  I have kind of a two part question.  I want her to have a cage mate, but she's "older" at almost a year and a half, so should I get her an older female cage mate, or does it matter?  I don't want to get a younger one and have her beat up on her (not that she's violent, but I'm still worried) or would it be better for her to live out alone?  She doesn't really enjoy being handled a lot, so I'm worried she'd be depressed and lonely since she wouldn't want to be held much.  Also, for the time being, should I clean the cage really well to prevent her from thinking Charlie is somewhere in the cage?  I had removed Charlie during her last hours because she was seizing and I didn't want her falling or Precious beating up on her so Precious didn't see her sisters death and I'm worried she might not know she's passed.  So to sum up, get a new cage mate?  Best age?  And clean cage thoroughly now to get Charlies scent out?  Also, please leave tips for introduction if you don't mind..."

Answer
I would get her a pair of younger rats. This way, the younger two will have each other since Precious is older already. If not, you will always end up with a lone rat and face the same problem.  

I would get young ones, around 12 weeks or so. This way they will have each other to chase around and play and wont get on the nerves of Miss Precious by bothering her all the time. She can decide if she wants to join in or not.  Also, having two rats, precious may not want to bully anyone since she will be out numbered.  

For intros, after quarantine, move the new rats and their cage next to Precious. Keep the cage far enough away that they cannot reach each other, or their tails etc...  When Precious comes out to play she can go and sniff them through the bars and vice versa. After a few days, put the baby rats in her cage while she is in their cage. They will pick up each others scents.   Watch the reacton Precious has toward the new rats and if you feel good about it, let them meet while you hold Precious. Also, be sure you have gained the trust of the new rats before introductions to Precious so they dont feel scared of both you and her.

I am sorry you lost Charlie. I am sure Precious is looking for her, most rats do, and some more sensitive rats have mourned the death of their cagemate to the point they fall ill from the stress causing their immune system to weaken and they may also get sick too.  As for cage cleaning, you dont really have to do it to remove her scent, it may be comforting to Precious.