Am I permitted to attend my niece's son's wedding, which will include the groom entering the women's hall and the bride wearing an immodest dress, bearing in mind the doubt regarding adherence to Islamic songs, and knowing that I have covenanted with Allah not to attend non-Islamic weddings? And am I permitted to use the death of my mother-in-law as an excuse not to attend, in order to avoid angering my sister?

Whoever is invited to a wedding banquet must accept the invitation, unless there is an evil that one cannot change. If the bride's dress is revealing her 'awrah, or if the hall contains women who do not wear the legal hijab, and the groom enters upon them, then these are considered evils that negate the obligation to accept the invitation. If it is highly probable that they will adhere to the manners of Islam, then go. But if it is highly probable that they are heedless, then do not go except for the purpose of denouncing the evil and offering advice. If you cannot denounce the evil, then do not go.