I presume the sister does this from the pew.
If there is a woman in the pews that is knowledgeable enough then she can answer. If not, the priest simple makes the appropriate responses.
While religious nuns have customarily set up altars even before Vatican 2, if there are not religious nuns at the church, it would be more appropriate for the priest to prepare the altar. However, I would not refuse to attend a Mass just because a woman set up the altar or made responses from the pews. If she actually acted as an altar server, however...