So I've watched FMA: Brotherhood (in quite a rush), and I can't seem to find an answer to whether Trisha Elric knew about Hohenheim's immortality and his past. Had he revealed it to her? Or was she aware of it somehow?
Trisha certainly knew.
In the beginning few minutes of episode 36 "Family Portrait", Hohenheim is speaking with Trisha. He calls himself a monster, reminisces about all the things he has seen, how he never grows old, etc. and Trisha seems to understand what he is talking about. Then Trisha arranges to have their family picture taken so they can remember them all together before she and the boys grow old. She also comments to Hohenheim that he isn't a monster, obviously knowing why he called himself that. Finally, Hohenheim prepares to leave his family and is about to walk out the door and Trisha asks if she should wake the boys. Hohenheim responds with a no, telling her he didn't want them to know about his body. So Trisha obviously knew what was going on.