Re: how would a clever DIL behave while shes living with the inlaws...
^ U pick n choose your own battles. Life is too short to make every little thing an issue. I agree with that. And I'm not too bothered by people whom I don't care much about or see on a regular basis. Maybe the offender will get exhausted once he/she sees they have no effect on you. But it's not always like that.
From my own experience, there are people who love to engage in fitnah/mischief...to the point that they have a sick systematic AGENDA to cause serious problems for you. And when you've tried all the other tactics such as staying quiet, or showing undeserved kindness, and they're still persisting........some straightforward communication needs to take place. The other person may not change with this discussion....but I feel that when you see a persistent wrong taking place....then you should try to do something positive about it. Otherwise, leaving things unchecked can lead to a vicious cycle in which there will be other victims.
Patience and tolerance are difficult, that's why they are such admired qualities in Islam. But then there is also the hadith, that if a Muslim sees something wrong happening, he should try to stop it himself. And if he can't do that, then he should contact someone who can help. And the weakest level is to pray for improvement if the above two can't be done.