Is the hostess considered sinful if she invites her friends to her brother's mixed wedding, which includes music, after informing them of that beforehand, knowing that she cannot refuse to attend because she lives with her parents? And is it obligatory for her to inform her relatives about the ruling on niqab, even though she is not a scholar and cannot present the argument sufficiently? And is she sinful if she cannot convince them to abandon the reprehensible things they invite her to?

It is not permissible to attend a wedding that includes forbidden acts such as intermingling (between sexes) and musical instruments. It is obligatory to avoid witnessing forbidden acts if you cannot remove them. It is not permissible to invite your friends to it, for Allah, the Almighty, says: "And do not cooperate in sin and aggression." The absence from attending or inviting can be justified by the presence of forbidden acts. If you are compelled to attend, there is no sin upon you. You must be strong in Allah's religion and fear no one. As for the niqab, it suffices to say that it is part of Allah's religion and the attire of the Mothers of the Believers, and you love to emulate them. Strive to seek Islamic knowledge to convince others, and silence is better if you do not have sufficient knowledge.