Is it considered a sin to bring in a foreign worker to repair the house while the mother and daughters are present in the absence of a mahram?

There is no objection to a woman working as long as she adheres to the complete, lawful hijab, provided there is no khalwa (seclusion) with a male worker. Khalwa is negated by the presence of a mother, or discerning sons or daughters, but it is not negated by the presence of a small child from whom one does not feel shame. It is permissible for a man to be in khalwa with two trustworthy women whom he respects.