tax basis on sale to family member

What is the name of your state? CA
This is a four part question. First, the history:
My parents placed their home in a QPRT, but only one parent survived the term. Hence, half the house was transfered to the offspring, and the surviving parent pays rent on that half, while the other half reverted to the trust upon the death of the first parent.
First, does the half that reverted back to the estate upon the first parent's death have a stepped up basis?
Second, if an offspring wishes to purchase that interest, does he inherit the property tax basis under Prop 13?
Third, if the offspring also wishes to purchase the remaining interest from his siblings, does the tax basis remain the same? What about the parent purchasing back the interest from the offspring?
Fourth, is the cost basis on the QPRT half the original cost basis or the stepped up cost basis? And if one sibling lives out of state, can he still shelter $250,000 in gains? I'm assuming that he cannot shelter $500,000 as his spouse was not part of the QPRT, correct?
Thanks so much!

[HomeGuru]

Please post in the Tax Law forum.