His dead brother texted me from an apartment

AITA for going through my husbands phone and finding out his dead brother is not actually dead. Okay so this is going to sound insane but please stick with me. My husband Marcus always told me his younger brother Dean died six years ago in a car accident. He said it so painfully that I never once asked for details. I just held him when he cried on the anniversary every single year. Last week his phone buzzed while he was in the shower and a text popped up on the screen. It said hey man can you send more money this month rent went up. Signed Dean. My stomach dropped. I told myself it was a coincidence, maybe a different Dean, maybe a scammer using his dead brothers name. But then another text came through. It said tell Sarah I say hi, she does not even know I exist which is probably for the best. Sarah is my name. I felt like the floor opened up underneath me. I grabbed the phone and scrolled through months of messages. Dean was alive. Very alive. Living two states away in an apartment that Marcus was paying for. There were photos too. Dean at a barbecue. Dean holding a baby. Dean looking exactly like the photos Marcus kept framed in our hallway of his supposedly dead brother, just older. When Marcus got out of the shower I showed him the phone with my hands shaking. He turned white as paper. Then he sat on the edge of the bed and told me the truth. Dean was not dead. Dean had gone to prison for a violent assault when they were in their twenties. Marcus was so ashamed of it that he told everyone, including me, that Dean had died instead of explaining that his brother was a convicted felon. He said it was easier to grieve a dead brother than to admit his real one was behind bars. I asked why he kept sending him money and talking to him in secret for years after we got married. He said Dean got out three years ago and has been struggling to rebuild his life, and Marcus felt guilty leaving him behind so he quietly started helping him without telling me because he was terrified of how I would react. I sat there trying to process that I had cried real tears at a fake grave for a man who was alive this whole time. We actually have a small memorial marker in a cemetery with Deans name on it. Marcus paid for an empty plot just to sell the lie completely. I asked him if anyone else knew the truth. He said his parents knew, his cousins knew, basically his entire family had been in on this secret for six years while I was the only one left in the dark. That hurt worse than the lie itself. I felt like an outsider in my own marriage, like I married into a family that decided I could not be trusted with the truth. Marcus kept saying he was protecting me from judging Dean unfairly, but all I could think was that he judged me as someone too shallow to handle the truth instead of trusting me as his wife. I left the house that night and stayed at my sisters place. I have not gone back in four days. Marcus calls constantly, says he is sorry, says he wants to introduce me to Dean properly now that the truth is out. of me wants to meet this brother who has been a ghost and a secret for years. Another part of me wonders what else in my marriage is not what it seems. If he could fake a death for six years and keep his entire family looped in on the lie, what other stories has he rewritten for my comfort. My sister thinks I should leave him completely. My mother says people lie for complicated reasons and I should hear him out. I honestly do not know anymore. Am I overreacting by being this hurt over a lie that was technically meant to protect me, or is this such a massive betrayal of trust that the damage cannot be undone.