Is a girl considered sinful if her family goes along with her in holding a mixed-gender wedding based on an agreement that was made, then they do not abide by the agreement on the wedding day, while also being extravagant in the preparations?

A Muslim is not held accountable for the sins of others. So, if your family commits an objectionable act, such as a mixed gathering, music, or extravagance, there is no sin upon you if you do not participate in it, and neither see nor hear it. It is forbidden to wear the hijab in front of non-mahram men. The suitor must not approve of his family's objectionable acts or help them in them, and it is forbidden for him to be present in a place where objectionable acts are committed. It is recommended to seek help from Allah and from virtuous and wise people for mutual advice with kindness.