I have two brothers, and I am the third. My future daughter-in-law is older than my second brother's wife. My mother said it was impossible. What fallacy is this? Ask the great god for guidance

In fact, nothing you said mentioned your brother's opinion and your father's opinion. Let me guess, in fact, they all support it. In fact, there are only two people who are against it, one is your second sister-in-law and the other is your mother. You don't know what your second sister-in-law thinks, because she never told you, that is to say, it is your mother who really opposes you. That's easy. As long as you convince your mother, she will not oppose you and will support you. You can tell her, first, you really love each other, second, even if the other person is old, there will never be any problem with seniority, and third, you can tell your mother that these are your own opinions. Why not ask your sister-in-law's opinion, and then encourage your mother to ask your sister-in-law. The reason why your mother complains is not that she doesn't support you. She should be a very easy-going person at ordinary times, so she is afraid of quarreling at home. As long as you promise not to quarrel, and take your future wife to see your second sister-in-law, and make a good relationship between them, when you really take your wife to see your mother, your second sister-in-law will help you.