How to value 12 unit building?

[AS Chow]

My brother and I own 25% each of a 12 unit building in SF.  Our sister runs the building and owns 50% and she wants to buy us out - she wants to keep the building, and I want to buy a house.

She got an appraisal report for 6.0m 

GRM on the report is 19.0

Indicated Overall Capitalization Rate of 3.1%

There was another appraisal done a year ago from a different firm at 5.4m with a 18 GRM.

I know there are more factors in pricing a building... 

I don't really know if I should take the buyout? Or ask for a second or third appraisal?  I want to be fair, and I think she does as well. We won't be paying a commission for the internal transaction.