In what form was ownership of the property being held (e.g., joint tenancy, tenancy in common)?
If you held title as tenants in common, then one-third of the basis for you and your sibling is what you and your sibling paid the third sibling in 2006.
The other two-thirds would generally be a carryover basis from your mother (i.e., your mother's adjusted cost basis) since the 1994 transfer appears to have been a gift. However, if the fair market value at the time of the gift was less than your mother's adjusted basis, then you would use the fair market value on the date of the gift (1994) to figure a loss (which loss would not be deductible since the property was held for personal use).
You would then need to add the cost of any improvements you and your sibling made over the years.
