Your sil knows how to manipulate the system. She apparently is in love with another man or doesn't want to be married to your brother anymore, so she committed adultery to get a "scriptural" divorce and still remain a jw.
I believe the latter to be the case. She certainly wasn't in love with the other man, given the fact she had just met him that weekend. I think another issue is that she was much stronger spiritually than him. He was inactive, and rarely attended meetings. To a certain extent, she was viewed as having an unbelieving mate, and was probably treated differently on that basis. Had he been an elder, or even a MS, the outcome might have been different.