When Tenants in Common sell a house can they only include the expenses of the sale for their basis?

I have 2 sisters, both clients, who were Tenants in Common for their Mothers house while she was living there. Last year the Mother moved into nursing care and the sisters sold the house and received a 1099-S for 1/2 the proceeds each. Since they did not put any $ into the house then they would have to pay Fed and state tax on the proceeds less only the expenses of the sale?