Familial ties bound victims, murderers and detectives in good and bad ways on Elementary Season 3 Episode 17. As Joan discovered, nothing makes us as crazy or hurts us or saves us the way family does.
Her mom had always been a formidable woman, so it was easy to believe her when she told Joan that she caught her brother kissing a woman who wasn't his wife. We had no reason to doubt her, although in retrospect, her comment about her husband's affair should have given us an inkling that she could have projected or been confused about what she saw.
Joan didn't see it either, but Sherlock gently guided her to that possibility.