How long had you known her before getting M? You had no idea that what she revealed about her past had happened? You never saw any odd behavior before the day at the beach?

It all sounds a bit strange to me, however I know there must be more to the story than what has been posted.

Back to the wedding, if your W and SIL had resolved their differences, why would you tell your brother that his W should not attend the wedding? Did you not know they had their problems worked out? Did he not know? Did it not come out during all the talk within the family? Perhaps you were referring to a different argument, IDK. It's confusing.

I hope she gets help with whatever happened in her past. Has she tried?

It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!