Why would a six year old boy always want to touch his nine year old brothers penis? They live with thier mother, who divorced when the six year old was four. I don't think the six year old has been sexually abused.
Six year old children are often interested in exploring sexuality. This has to do with curiosity and not with sexual abuse. Ask the nine year old boy to be dressed and get the six year old interested in something else, if you want to stop his behavior.