Is a woman permitted to refrain from visiting her husband's mother (mother-in-law) who mistreats her and her child, while showing favoritism to the other children and grandchildren?

There is no harm in reducing visits to the husband's mother due to her harshness, and there is no sin in doing so. However, it is preferable to be patient and forgive her missteps, for pardoning an offender increases one's honor, as stated in the Hadith: "Allah does not increase a servant by forgiving except in honor." And responding to evil with good brings affection, as in the verse: "Repel [evil] with that which is better; then, suddenly, the one whom between you and him was enmity will become as though he was a devoted friend."