nuestra feminine if we do not know the gender of the people the speaker is referring?
Ok, I must confess, at first I thought the question wouldn't make sense, but it does and actually it's quite interesting.
In Spanish adjectives, possessives pronouns, and so on are declined according to the noun they qualify. In this case, nuestra is qualifying madre, which is always feminine (unless..., no, always feminine).
Let's compare with other languages, (e.g. English). In English your possessive pronouns only refer, if at all, to the gender of the object which possesses. Thus in
or, for sake of generalization to plural, in
the only reference to gender and number is on the object that possesses the mother or the grandmothers, Mary in this case. This means, the possessive pronoun detects if the subject here is a Mary or John Smith (which implies her should be replaced his) but it doesn't deliver information whatsoever about the object that the subject possesses (if it's a mother, or a father or two grandmothers or 1024 great-great-...-great-grandpa's, that doesn't affect the possessive pronoun.)
However, Spanish is –as many other languages are– endowed with an additional gender and number slot: the noun that is "possessed"
means our mother but the gender information there refers to mother, not to us.