Will the sister be sinful for holding a wedding party with music and makeup in order to bring happiness to her parents and avoid people's talk? And what should her sister do about that? And is she obligated to attend the party? And what is the ruling on wearing party clothes?

It is not permissible to bring joy to parents through disobedience to Allah. The wedding day becomes an ill-omened day, not one of gratitude, if it includes singing. The prohibition of musical instruments is indicated by the Prophet's (peace be upon him) saying: "There will be people from my Ummah who will consider lawful illicit sexual intercourse, silk, intoxicants, and musical instruments," and his saying: "Two voices are cursed in this world and the Hereafter: a flute at a time of blessing, and wailing at a time of calamity." The scholars have unanimously agreed on the prohibition of listening to musical instruments, and only the use of the duff (tambourine) is permitted. If your sister insists on disobedience, refrain from attending if possible. Otherwise, attend in a place where you cannot hear the forbidden amusement. The clothing must cover the awrah (nakedness) between the navel and the knee for Muslim women, and what is apparent for working women among disbelievers.