What do I enter for Unrecaptured Section 1250 Gain of my siblings?

You'll need to use Worksheet 2 for Line 11 of Schedule D Tax Worksheet - Unrecaptured Section 1250 Gain  to calculate the unrecaptured section 1250 gain that should be entered here. Fill out the worksheet for each sibling. Then add up Line 5 of each worksheet and enter the total as the unrecaptured section 1250 gain of your siblings. Don't include your own unrecaptured section 1250 gain.