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