How can one deal with the fiancée's mother who rejects her daughter's commitment to hijab and niqab, and encourages her not to adhere to Islamic guidelines?

The mother should ideally rejoice at her daughter's commitment to the hijab. The daughter, in turn, must be patient with any harm inflicted by her mother and must not obey her in disobedience to Allah, for "there is no obedience to a creature in disobedience to the Creator." Try to convince the mother that the hijab is a form of protection and honor for women, not a hindrance to their lives. We advise expediting the marriage to avoid the mother's negative influence, while emphasizing that engagement does not permit looking or private seclusion.