Yes, you will divide the cost basis 5 ways and the proceeds 5 ways.
You need a reasonable method of determining the FMV that the IRS will believe if audited. The best thing to do would be to contact an appraiser. They may be able to do a retroactive appraisal based on historical records and the photos from the real estate listing. If the market was stable between August and December, the selling price may also be a good indicator of FMV, but if there were dramatic price changes in the market, there might need to be an adjustment. An appraiser can give you advice on that even if they don't do a formal appraisal.
