If the daughters were on the deed to the property before the parent passed away they may not be eligible for a step up in basis at death except for the fraction owned by the mother.
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https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/property-basis-sale-of-home-etc/prope...
Disclaimer: Not a tax professional. Information gathered from internet links. Anything dated in June 2019 was posted in prior years and is before the 2019 limits and changes.
