You have a complicated tax situation and I suggest you consult a local tax professional. You did not inherit your share in the property; you received it as a gift - from your brother who himself had received it as a gift. This IRS Q&A will give you an idea of the complexities involved in calculating the cost basis of gifted property:
https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/property-basis-sale-of-home-etc/prope...
You need to determine the cost basis of your share so that you can pay the correct amount of capital gains tax.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
