It is found in The Vessel in a room next to the catacombs, on a table.
I believe you're underestimating Brother Alders, Richter, and the hold Martin still has on him. Martin was weak and so is Alders. But there is a simple way to know the truth. Take Alders outside with Sister Harper. Ask the two of them to demonstrate their devotion to the family. I've no doubt Sister Harper will prove what Anders cannot.