Is the mother considered sinful against her daughter if she does not stand with her in her problems with her in-laws, in order to preserve her home?

Reconciliation is one of the greatest acts of devotion, especially among relatives, for Allah Almighty says: (No good is there in much of their private conversation, except for whoever enjoins charity or what is just or reconciliation between people. And he who does that seeking means to Allah's approval - then We will give him a great reward.) Therefore, try to reconcile between your daughter and her husband with wisdom and without bias. If she is in error, advise her to be a good companion. If she is wronged, urge her to be patient and repel injustice from her with wisdom.