Did I err in my conduct by asking to postpone a visit from my in-laws? What is my legitimate right, according to Sharia, to preserve my dignity with them? And is it permissible for me to limit my interaction with them?

You are not mistaken; rather, you are benevolent and forgiving towards your husband's family's mistreatment, and he has no right to punish you. Treat your husband with kindness and overlook his shortcomings. Your kindness to his family is a completion of your kindness to him. Agree with him that these matters should not be a cause for disagreement, and be sure to maintain ties with his family to the extent that no harm results. Repelling evil with good brings affection, and forgiving the wrongdoer increases a Muslim's honor and earns Allah's forgiveness.