Sounds like that may have been a trigger for her. The dynamics in her family may have stunted her emotionally as a child. Has she ever stated how her childhood was? The trauma to her which would have stunted her emotionally had to have happened as a child/teenager. Did the parents favor the other sister more than your wife? Where the parents emotionally absent from the children?
MLC isn't easy for anyone. Your wife has to find herself and grow up. You will need to find a way to give her the time and space she needs. Yes, she will try your patience, but dig deeper for patience. If you aren't sure what to do about something, do nothing. Sit quietly and the answers will come. If you have questions, come here and we will help you.
Keep a close eye on your finances, bank accounts and credit cards. If she is in MLC, she will eventually begin spending like water to self medicate herself. Depression is the main ingredient of MLC. Do not try to tell her that she is depressed...she will deny it. Learn to just listen and if she wants a response from you, she will ask you. Be a friend.
Keep the focus on you and your children. They need you now more than ever.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.