My mom has it, and yes autoimmune diseases can be hereditary. My mom has oral lichens planus. She uses "Magic Mouthwash" which is prescribed and made special for her. It has hydrocortisone for Inflammation, lidocaine for pain (numbs) and something else in it. She uses only natural tooth pastes- regular ones are too harsh. Tom's of Maine is a good one, otherwise she is currently using a Biotene toothpaste I think. She gets the absolute softest toothbrushes she can find. Uses reach gentle care dental floss. And has found a dentist that knows what it is and knows how painful it is and to be careful. She has to go in every 4 months instead of 6 for cleanings because her gums cannot handle any plaque buildup.
Bottom line- when she is stressed her lichens planus is terrible. She has learned to manage her stress to keep it to a minimum. It takes a long time for her sores to heal.
She also has a dermatologist she sees that knows how to treat it.