Must I attend the wedding of my niece who did not attend the wedding of my daughter, knowing that my family refuses my going to that [event]?

Invite your sister and try to find out the reason why she did not accept your invitation, for she might have an excuse, or she might not have approved of your invitation being conveyed through her husband. Address the reasons so that Satan does not find a way to sow discord between you. If she has no reason, do not treat her in kind, for the one who maintains ties is not he who reciprocates, but rather the one who, when his kinship ties are severed, reconnects them. Seize this opportunity to attain high ranks in maintaining kinship ties, and do not pay attention to those who wish to prevent you from connecting with her. If your parents were to forbid you from doing so, their obedience would not be obligatory, for accepting an invitation to a wedding feast is obligatory according to the preponderant opinion.