I shouldn't be sarcastic--it's a real, and potentially painful issue, if the intermediary spouse (sibling to one, ex to the other) chooses to make it one. Here's what I mean:
Dear Carolyn: My husband says that since my two brothers divorced, their exes are out of our family. I am nice to my brothers’ new girlfriends, but they were both married for 20 years, and I think of the exes as my sisters.
Also I’ve known my husband’s brother’s ex-wife since junior high school — longer than I’ve known my brother-in-law. We still hang out, and I consider her a good friend.
My husband says my loyalty should be with the new women and none of the exes.
I don’t want to be thought of as disloyal, but I have a very hard time with this situation. I don’t want to have to pick one side.
Carolyn gives a good answer, as usual: I don’t want to take sides, either, so I’m going to have to figure out a nonpartisan way to point out that your husband is being a complete tool.
In short, she says, unless the divorce was caused by "inexcusable" behavior on the part of the ex-in-law, the middlespouse (or middlespousal sibling) has the responsibility to be mature, polite, and respectful of the other parties' continued affection for one another. In Carolyn's more succinct words, "Memo to your husband: Where one spouse shows integrity [by treating friends with the same affection and respect, regardless of their familial status], it's on the other spouse to show some respect"
Things get especially thorny when it's the middlespouses themselves who caused the breakup of the marriage. I know someone for whom this is an issue: her brother was unfaithful to his wife (beloved by the extended family), leading to a divorce , and is now married to the other woman, whom none of them enjoy. She refers to both women as her sister-in-law, sometimes qualifying the ex as an ex, sometimes qualifying the current with a scowl. An ever-thickening plot...
How do you handle anger at a sibling, whom you love, for breaking the ties that bind you to the sister you never had? Worse, how do you forgive your brother for being the jerk that cheated on your friend? Is blood thicker than water even when it seems to be just messing with everyone?
Possible solution: brother and new wife move out of state, returning for occasional visits, while ex-sister-in-law remains nearby, coming over often for visits and graciously and lovingly accomodating niece and nephew sleepovers.
Almost everyone kind of sort of wins!