Praise be to Allah.
What the girls family wants to do is undoubtedly haraam and is unacceptable. It is not permissible to please people by doing something that angers Allaah. We do not advise you at all to start your married life with haraam actions. The Muslim should disassociate himself and his family from the idea that his wife is a cheap product to be looked at by all and sundry when she is wearing all her make-up and jewellery etc.
We advise you to deal with the situation by doing the following:
You should advise them, politely, and explain to them the Islamic ruling concerning what they are planning to do. Warn them of the wrath of Allaah, and explain to them that music and mixing are haraam. Tell them that they could still have a successful wedding without these haraam things, and it is not in their interests at all in this world or in the Hereafter to respond to the blessing of Allaah, i.e., their daughters marriage, by disobeying Him and going against His commands and doing things that anger Him.
If that does not work, look for some wise people among their family and relatives, and from your own family too, who they think highly of and who you hope can help. Perhaps Allaah will decree that you will find a way out through them, and they will give up these evil things even if it is through pressure and embarrassment.
If that does not work, look for intervention on the part of someone who is knowledgeable and wise, whom they respect and look up to. Perhaps he will make them feel ashamed, or will be able to convince them that what they want to do is wrong, so they will give it up.
If none of this works, perhaps you can threaten them with divorce or separation. They might pay attention to that, because it would make them look bad in front of other people, so they will give up that which has been forbidden by Allaah. Perhaps leaving a long time between the nikaah (marriage contract) and the wedding party will have the result of convincing these people.
If they do not respond at all, then we would seriously warn you against getting involved with these people. But if the girl is religious and good-mannered, and she does not agree with what her family are doing, and you and she are able not to be present when the haraam things are happening, and you can leave the party when they start doing things that anger Allaah, and also announce your objections and state that you are disassociating yourselves from what is going to happen, (this is fine). Remember the words of Allaah (interpretation of the meaning): then sit not with them (but if you stayed with them) certainly in that case you would be like them [al-Nisaa 4:140]. And the Prophet (peace and blessings of Allaah be upon him) said: Whoever among you sees an evil action, let him change it
And Allaah is the source of help; to Him we complain and in Him we put our trust.
And Allaah knows best.