Things kinda taking a toll here in Portugal. Everyone's quarantined at home for about 3 weeks now and still there are cases where they only started showing symptoms after 25 days.
She probably was a great mother and neglected herself as a wife. First family, second love life. Your BIL lost interest, maybe saw his wife let herself go over the years and went after someone...
She is going to cheat on you while you're a work with a classmate that has a car, mark my words. It might not be tomorrow, nor next month, but two years is a long time.