Matres et dominae is the Nominativus plural. But may be to the mother and wife (Dativus, sing., fem). It is the "title" of the woman of the house, I believe. Mater et domina. The mistake is sure related with the "et". I incline to the singular. "To the mother and wife", or "In the honour of the mother and wife". It depends on the original.
Yes, the requester must have meaning "matri et dominae", but, as the translation has been already accepted in English, I'd to "correct" the source to "matres et dominae" in order to match the English translation.