What is the legal ruling on a girl marrying someone she agrees to, if her guardian (father) refuses and wants to marry her to someone else? And would the eldest son who took responsibility for her care be considered disobedient to his father if he marries his sister to the one she desires?

It is not permissible to marry off a girl except with the permission of her guardian (wali), and any marriage without a guardian is void. The father's guardianship is forfeited and transfers to the next closest guardian if he withholds his daughter, meaning he prevents her from marrying a suitable partner without a valid reason. The reason for the father's prevention is examined; if it is considered valid, his objection is accepted. Withholding occurs if a mature, sane woman requests to marry a suitable man and the guardian refuses, unless the groom is unsuitable. The withholding must be proven before a judge for him to marry her off. If the girl designates a suitable man and the father who has the right to compel (al-ab al-mujbir) rejects him, he has the right to do so, according to the more sound opinion, because he has greater foresight. As for a guardian who does not have the right to compel, he does not have the right to marry her to someone other than the one she has designated. If you are a suitable partner and the guardian rejects you, but the girl designates you, then her guardian is withholding.