Tax question

If there is a different group, forum more appropriate for this please advise.

Real estate, a condo is bought in 2000 by a not very healthy spinster. We’ll call her Gertie. Gerties mother has been giving Gertie financial help for years and Gertie decides to file a quit claim deed in 2001 to her mother in case she dies her mother will own the condo without dealing with probate. Years later her mother now well into her eighties and Gertie decides that Gerties sister is more likely to outlive her and removes her mother from the deed and adds her sister in 2008. Then about nine years later in 2017 Gertie dies. Gerties sister under her right of survivorship sells the condo for $120,000.
It has been explained that the tax burden now on the sister is the difference between the” basis” and the amount it was sold for.
Tax question for Gerties surviving sister; How does she determine the basis?
Our tax guy is sure sisters taxable gain is from original 2000 purchase price about $70,000.(basis) That would incur a tax on $50,000.
Sisters opinion is she was only added to the deed in 2008 at which time the value of the condo was estimated to be $100,000. (basis) Resulting in a tax liability on $20,000.
Opinions?