What is the ruling on wearing a "skirt and tailored jacket" instead of the Shar'i jilbab at one's sister's wedding, given the presence of the sister's husband in the hall, and knowing that the mother and sister are pressuring for it?

You are not permitted to obey your mother by wearing tight clothes and dancing in front of your sister's husband, as this constitutes disobedience [to God]. You must not display your adornment in front of non-mahram men or dance before them. Allah Almighty says: "And Allah and His Messenger are more worthy that they should please Him if they are believers." (Quran 9:62). Try to gently explain the religious ruling to your mother, and if she becomes angry, there is no sin upon you, for a woman dancing in front of non-mahram men is a universally condemned evil.