Possible Clues:

• Condition whose therapy may include hissing like a snake
• Beth for Bess, e.g.
• Non-standard pronunciation one's detected in old recording
• Make a sinner thinner?
• An unmarried girl with this is just a myth
• Be challenged by one's sisters?
• Struggle with assessors?
• Imitate Sylvester the cat
• Cindy Brady feature
• Baby-talk characteristic