doesnt know her own sister

Question
Hi , I have 3 female rats all brought at the same time , lived together and are now just over 1yr old. One of them had to go for a lump removal and was  from the others for a day . When I got them all out together one of the other 2 won't go near the  oparated one and runs away ( Evan jumping on the floor ) to escape ? I don't understand her behavior, surley she should know her own sister ?
What can I do to get them back together I don't want to keep any one seperate as it seems unfair .(the other 2 seem to be fine together)

Answer
She does know her sister, but she's a little afraid because her sister suddenly smells unfamiliar, due to the surgery environment she was in and perhaps the medications she's on.  This is very common.  Has happened every time I took one of my own rats for surgery.  

As long as her sister doesn't show aggression towards her, just give it a few days and all will be back to normal. M

Please let me know if you need help with anything else.