Tranq, the title of your post has me thinking.
This is surely a sad thing and will last for quite a while...it's like a death in the family...but does it really mean you can't trust <u>anyone</u> (men)?
you'll never know what went on between your sister and brother in law...something didn't work that made him stray....maybe his lack of committment certainly, but maybe something else was broken in the marriage.
keep an open mind and even though your sister has been hurt, try to stay neutral if at all possible., otherwise she really will never trust anyone again and will have a difficult time forming a new relationship.