Why dont Rodney move out of Delboys place and get a place of his own with Cassandra and be his own boss and any money that comes Rodneys way would be alright for him and Cassandra. and get a company and run it under Cassandras maiden name then Delboy will not know who the persons are.
I think Rodney is supporting his brother .. He sees Del as a father figure as he is the one who brought him up so he sticks by Del. Also i don't think Rodney could cope with the pressure of being his own boss - he just doesn't seem that type of person.