Your IRA is allowed to purchase a rental property. Your daughter would like to pay rent and live there. She is a disqualified person and therefore it would be a prohibited transaction. However, your niece indicates she would like to live and pay rent. Since she is not a disqualified person, it is permissible and not a prohibited transaction.
Your son would like a loan from your Self-Directed IRA to help with college expenses. Again, he is a disqualified person so your IRA should not make the loan. Your brother also asks for a loan – for any purpose – he is not disqualified therefore your IRA can make the loan.