Step 1: From "A is B's brother" — A and B are siblings.
Step 2: "C is A's mother" — therefore, C is also B’s mother (since A and B have the same parents).
Step 3: "D is C's father" — meaning D is the grandfather of A and B.
Step 4: "E is B's son" — E is the child of B.
Step 5: Since D is B's grandfather, D will be E's great grandfather (one generation more up from grandfather).
Thus, the correct relationship is $\mathbfGreat\ Grandfather$.