Who is Mary Jane Mayfair from Taltos: Lives of the Mayfair Witches and what is their importance?

Mona's cousin, Mary Jane Mayfair, is very much a country girl. She doesn't become an integral part of the family until after Mona learns of her pregnancy, and Mary Jane is the one who perceives something isn't quite right from the start. When Mona asks Mary Jane straight out if she believes there's a problem, she refuses to place her hands on Mona's belly..... believing that doing so would be the wrong way to practice witchcraft. Whe Mona finally gives birth, it is Mary Jane who switches the baby in order for the doctor to sign a birth certificate.