Is it considered undutiful for a son to refuse to bring his wife to visit his mother, if the mother insists on that despite recurring problems, and threatens to be displeased?

The approach you followed in having your family visit your relatives and staying with them every weekend is good and intended by the Shari'ah to strengthen the bonds between in-laws. If there are problems between the wife and the husband's mother, care must be taken to preserve the kinship by marriage, as Satan seeks to sow discord among people. The husband must be wise and play the role of a reconciler, removing the causes of problems by agreement with both parties. It is permissible to convey words that bring affection between them, even if they are not entirely true. The wife is not obligated to visit her husband's family or obey him in that regard, but her visits and overlooking minor faults will greatly increase her reward. If the wife insists on refraining, the husband is not permitted to force her, and his disagreement with his mother in this matter is not considered disobedience to parents. Obedience to parents is not absolutely obligatory; rather, it is obligatory in matters that benefit them and do not harm the child.