Question What's the difference between "custos fratris mei" and "meus frater custodis" or do they both translate (to English) the same way?
Answer The first phrase means "the guardian of my brother"; the second phrase means "my brother of the guardian." The second phrase seems nonsensical.
I suspect that you are trying to g1et at the text in Genesis 4:9, which is often rendered in English something like: "Am I my brother's keeper?" St. Jerome's Latin Vulgate for that passage is: "Numquid custos fratris mei sum ego?"