Do the grandfather and grandmother have the right to prevent their orphaned granddaughter from marrying?

Some relatives may object to a marriage, and this objection does not affect the validity of the marriage as long as it has the guardian's consent. However, if the objection comes from the woman's guardian for a valid Shar'i reason, then the marriage is not valid. Guardianship in marriage is exclusive to the 'asabah (agnates), who are the male relatives from the father's side, such as the father, then the paternal grandfather, then the son, then the brother, then the nephew, then the paternal uncle. As for relatives from the mother's side, such as the mother's father (maternal grandfather), they are not guardians. So, if the "grandfather" in the question refers to the mother's father, he does not have the right of guardianship, and the marriage is valid without him, as long as the woman has a guardian from her father's 'asabah. However, if it refers to the father's father, he is a guardian, and the marriage is not valid without him, unless he rejects the marriage to a suitable suitor without an acceptable reason. In this case, his guardianship is forfeited because he is an 'adil (one who unjustly withholds consent).