Tax calculation

I am preparing my mother's tax return.  She was 94 December 31, 2018.  I have prepared the return but the tax calculation seems way off to me.  Her taxable income was $70,964 but Turbo tax only calculated a tax of $5,049.  She sold a piece of land that led to a $70,000 gain.  She held the land for 48 years.  Every tax rate I have seen shows at least a 15% tax rate.  Concerend that TT is not calculating tax properly.  I am thrilled if it is and I am missing a rule concerning her age or length of time land was held.