I just want to say, either Astrid is stupid and I don't pity her at all or there is just a really big loophole in the Dark Brotherhood questline. So we all know that she makes a deal with Commandor Maro selling you out and saving the Emperor's life. But my question is: Why would you make a deal with someone when you've killed their son? If they didn't know before it would be pretty obvious they would know now. I wouldn't blame anyone for wanting vengeance for that.
Astrid didn't kill his son, you killed his son. She offers the life of his son's killer inexchange Maro lays off the DB.
Say your commander Maro. "I can give you what most desire, the life of your son's killer, all I ask in return is you leave the brotherhood to do its business." What Astrid likely didn't know was Maro had already learned the pass phrase to enter the sanctuary.(which he gives to the player if he takes the destroy the DB path).
Commander Maro can't be sure you killed his son Gaius (unless you commited the murder in plain sight), but you are right, it would all be a bit too accidental. Astrid even admits herself that she was foolish, and wanted nothing more than to keep her little family together.
Sure he can because Astrid
told him you did it.