My cat hates my other cat after it went missing for two months?

Hi, I lost a cat two months ago and and thankfully found him, but his sister who he's been with since birth now hisses and growls at him (very uncharacteristic for such a loving cat). Both are the FRIENDLIEST cats you will ever meet. We got the missing one back from an animal shelter. Could it be the sister cat doesn't like his new "smells" from the animal shelter? Is it a territorial thing since the sister cat gotten used to being the only one? I don't think she even recognizes him. Is this normal for found cats? If so, how many days will it take to get back to normal? The sister cat wont stay in the same room with her brother. (NOTE: it IS the same cat from before no doubt about it and neither are feral)