If this is a short sale (and it sounds like it is because you mentioned his bank accepting the offer) then you should read all paperwork very carefully. Generally speaking, the transaction you are describing is not allowed because of the arms length clause in 99% of short sale approval letters/processes. The arms length clause basically means that the parties are unknown to one another. In addition, if the thought is for your step-brother to "rent" from you after close of escrow, that too is generally not allowed. If you decide to proceed, and the approval letters or other documents contain clauses that prohibit your situation, then you are committing fraud.