When Jake and Don visit Fiona in the hospital, wouldn't her stepmother object to their (brief) display of affection? Seeing as she's gone to rather extensive lengths to bar Richard from Fiona's life, wouldn't she notice and be disapproving? I know they deny it a bit later and there was a lot going on in the room, but Fiona's stepmother seems almost entirely absent from that scene.
Maybe she just figured they were brothers at first. Or really close friends. Or she just didn't care enough right then.
Actually, they weren't keeping Richard from seeing Fiona because he was gay, it was because they didn't want him telling her the truth about what happened to her mom.