I am aware that a wife can claim Mehr from her husband estate in case of his death but what about the case when she abandons the house herself, takes kids with her and leave her old in-laws stranded. She refuses to let in-laws meet their grand children. Will in this case she still can get Mehr? Is there a condition where she can loose all or part of her Mehr after her husband death because of her irresponsible behavior?
Mehr is wife's personal property, her behaviour described in the question does not change her ownership over her personal belongings.
You may argue in front of the judge (qadhi?) about the inheritance the wife is supposed to get because of the death of her husband. However, inheritance is separate entity from mehr. That's,
- Mehr: wife's personal property which was granted her by her husband at the time of marriage agreement.
- Inheritance: portion of deceased husband's property given to wife.
Wife could have offered (or asked) to return mehr (in part or full) if she divorced her husband by her will, but that was not the case according to your question.
So, mehr should be given in full* to wife and wife's behaviour towards her old in-laws does not affect her ownership over mehr which is her personal property.
* Assuming that deceased had not given mehr before passing away and they (the wife and the deceased) had not agreed to waive the mehr later during their marriage.