Here is the full list of all 16 Brother Brother achievements.
You killed the Dire Wolf, Gradius!
You've got a Faerie!
You killed the Fire-Lord, Moltex!
You killed the Water-Lord, Aquiux!
You killed the Forest-Lord, Terex!
You killed the Dire Wolf, Oryx!
You killed the Dire Wolf, Karma!
You killed the secret Spider Queen, Tulana!
You killed the Mad King Tobias and saved your brother!
You talked to Ward, and he's now at peace with his issues.
You found all the extra hearts!
You found the Fire Ring, fire away!
Beat the final boss with only 3 hearts. Not for the light hearted.
You've read the hidden journal. Who is it talking about... and where are they now?
You beat the last boss faster than I could. FANTASTIC!
You blew up a frog. Maybe you should talk to someone about that.