The Second DIL (and Third, Etc.)

I think it all depends on how it's presented and handled.

I don't think it's a requirement that a parent like an AC's partner. I do think it's a requirement to at least be civil toward them.

I have no issue with a parent continuing to have a relationship with an AC's former partner - as long as that is kept separate. For example, it would not be appropriate to say to AC in front of their current partner, "Oh, did you know what xD/SIL is up to lately?" or to invite the AC's ex to a function that will be attended by the AC and the current partner.

AIL kicked up a fuss because she learned that Hubby was talking to her XH. Hubby still considers him an uncle. Well, according to AIL, that was tantamount to betrayal - the fact that they weren't even talking about her notwithstanding. This is one reason why she is Aunt Who We Don't See. (She is a sister to FIL. Go figure.)

We loved our first SIL like one of our own an dwe were gutted to discover he had been cheating almost continually since the wedding.We still see him and are civil to him in fact when he has a problem its my OH he calls .My daughter stayed friends despite his treatment of her for the sake of the chidren .It works fine for them ,he visits with his wife ,Stepchildren and new baby and the kids accept its just an extended family.My 2nd SIL took a while to get used to ,in his own words hes "an arrogant wee sod" ,an only child used to getting his own way .Not the most sociable of men but he makes her happy and the kids love him.He has improved over the years though and we've grown to maybe not love him but like him a lot.Now my sons ex is a whole different kettle of fish!

I agree with what Smileless2012 says about a preference not having to be made obvious.
It is a mother's own interest to make the new daughter or son in law feel welcome, and not second or third best.

A preference can exist but that doesn't mean it has to be obvious. There's a big difference between having a preference and making a future or new d.i.l. feel unwanted.

If there are GC involved then it would make sense for the GP's to want to maintain a good relationship with their AC's ex.

The maturity of the new d.i.l. is important here. Her predecessor may have had a good relationship with her parents in law for some time and no longer being married doesn't mean an automatic exclusion from the family she's been a part of, and to a certain extent remains a part of.

I have no daughter in law , two daughters, two son in law’s and my son in law who isn’t . My younger daughters first husband . Nothing could change my affection for my son in law who isn’t, his second wife accepts this as does my daughter.

A mother in law can't help liking one daughter in law more than another, but if she shows her preference for an ex she may well see less of her son as a consequence.
Her son will want his mother to get on with his new partner, because she is his choice. If his mother makes his current partner feel unwanted, she will not want to visit which is bound to affect the number of times that the son visits.

An interesting thread Starlady.

We thought we had an excellent relationship with ES's wife which was why she was able for so long to manipulate him until we were eventually cut out of his life. We had no idea what was going on until it was too late.

We had an OK relationship with DS's ex, we were never close but enjoyed one another's company. There were faults on both sides which resulted in their divorce so we wished her well and hope that she and our DS may eventually find the happiness they couldn't achieve with one another, with someone new.

If DS were to re marry or just have a committed relationship in the future I wouldn't be worried about trying to cement a relationship with her, just take things as they come and always be prepared to meet her half way.

I just think that family dynamics have changed and that for many of our AC and their partners the importance of getting on with and having a good relationship with their in laws isn't as important as it used to be.