Anonymous wrote:1. both mother and daughter are irresponsible, in this case the apple did not fall far from the tree.
2. no reason to pay for the wedding, most likely it won't last, niece is there for the party, clearly does not understand that the wedding itself it's just a day and has really no bearing on the rest of her life. getting married is much more than having a fancy party. the mother should pay for any deposit the couple made, since she did offer to pay. she should explain to the couple that she cannot pay for the wedding. niece can have a small gathering at home with close family and friends, she can do that with few hundred dollars.
3. OP should stay out of it, all people involved seem immature and irresponsible, and you cannot beat sense into people who do not have it. life eventually will.
I forgot to say that the best gift OP's sister can give her daughter is to teach her to grow up and to be financially responsible, or the daughter will end up broke (and divorced) at 30 and will carry debts for the rest of her life. she should resist the daughter's demands, explain the reasons and stick to her guns no matter what. what the daughter is saying about the agreement sounds like a pile of BS, OP's sister should not have paid the deposit. this will end up in a financial and emotional mess. what a lovely way to start married life.