I would follow the instructions for reporting nominee income, even though it doesn't really meet that definition.  I wouldn't bother with 1099 filing; maybe attach a statement explaining the situation, if the amounts are large enough they might trigger a CP2000.  I would be particularly concerned about the stock sales.  Suppose they sold shares for $10,000 with a basis of $10,200.  If the sister whose SSN appears on the 1099-B reports only $5,000 of proceeds, IRS is not going to recognize it as half the $10,000 they know about.  So they are going to propose an assessment of $10,000 LTCG, if there is no basis reported because it's pre-2011 investment.