Is a woman who wears the niqab considered sinful if she removes it to please her family and fiancé on her wedding day? And does the sin fall upon those who coerced her into it if she was compelled?

The most preponderant opinion is the obligation to cover the face. Based on this, you must adhere to covering your face and not obey anyone who tells you to uncover it, whether at a wedding or otherwise. There is no obedience to a creature in disobedience to the Creator. The Prophet, peace and blessings be upon him, said: "There is no obedience in disobedience to Allah; rather, obedience is in what is right." Disobeying parents in this situation does not entail any form of disrespect towards them.