Gifts are free of gift tax up to a limit. But the sale is taxable to you. If you mother's tax advisor told her that you wouldn't have to pay capital gains tax when you sold the property they told her wrong.
The tough part is figuring out the profit. Since it has been in the family 40 years, it has probably passed through a couple of generations. So you need to figure out the ownership chain, dates, and whether the change of hands was due to inheritance or gifting.
It matters. There will be long term capital gains tax to pay. But determining the "cost basis" is very important and that will take some research.
