Taxes on sale of large collection of inherited art

Hi: My parents inherited a large collection of art in1971. Dad died 2014. Us siblings sold the art for 905,000.00 after fees from auction house. Mom is incapacitated and still alive. How do we figure what taxes she owes as we have no idea what the step up in basis is from 2014 when Dad died. There is no way to have over 300 items assessed. Thanks for any info. She lives in the state of California. The only thing i can figure is to do a reverse inflation calculation of what 905,000.00 was worth in 2014 and subtract that from total.

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