Is it wrong for me to feel that as a wife, my primary duty is to my husband, and not necessarily to maintain a constant close relationship with my mother-in-law?
No doubt, your primary duty is to your husband, but never cause your husband a harm or insult or disrespect through ignoring his family. Your positive attitude to your in-laws reflects on the life and happiness of your husband. Try your best to keep your husband happy and respected in his family.
Maintaing good relationship with your in-laws is part of the happiness and respect of your husband.
Wassalam..