pentheraphobia (noun) 

Pentheraphobia (PEN-thur-uh-foh-bee-ah) is the fear or strong dislike of one’s mother-in-law, a real – and surprisingly common – psychological affliction. The word is derived from the Greek “penthera” (“mother-in-law”). People with this phobia may also have trouble with novercaphobia (fear of one’s stepmother), vitricophobia (fear of one’s stepfather), and/or soceraphobia (fear of one’s parents-in-law).

(I challenge you to use any of these words in casual conversation.)