Is it permissible to shake hands with the maternal aunts of one's mother and the daughter of one's grandfather's wife, knowing that one was ignorant of them being unmarriageable relatives (mahram)?

It is forbidden to shake hands with the daughter of the grandfather's wife, because she is not a mahram to him; she is a stranger to him. As for the mother's paternal aunt (amma), she is in the position of a paternal aunt to the children, and it is permissible to shake hands with her because she is a mahram.