Is it permissible for me to hand over a sum of usurious loan money from my brother to the landlord, knowing its source?

The prohibition of an interest-based loan is related to your brother's liability, not to the money itself, so it is permissible to deal with him in this regard. Delivering the amount to the homeowner does not constitute aiding your brother in usury, therefore, there is no blame on you for acting as his agent to hand over the amount to the homeowner.