Is it permissible to marry the daughter of a son's wife without the approval of the son who is her guardian? And is the son's objection to this marriage considered an act of disobedience?

The daughter of the son's wife is not forbidden (to marry), nor is it unlawful to marry her. Many scholars hold that guardianship in marriage is established by a will. If the guardian's refusal to marry off the girl is due to a valid Shar'i reason, such as the suitor being unsuitable for her, then there is no disobedience in that. However, if the refusal is out of stubbornness and without a Shar'i justification, then the guardian has acted improperly, and the girl may refer her case to the judge to marry her off.