I suspect trouble on this whole thing.

There should probably have been K-1's issued for all the years rented as well. I do not know how they handled the shared ownership of rental property. I have seen some do it as a partnership and issue K-1's and some just have one individual report all the income and share it (Not best practice) and worse, I have seen some not report it at all.

But the capital tax gains is taxed separately from regular income. It is not added to regular income and then taxed as part of regular income.

https://www.nerdwallet.com/blog/taxes/capital-gains-tax-rates/

This has a quick calculator to figure your capital gain taxes. Take a best guess and enter the numbers on the worksheet about halfway down the page.