When the younger brother of the mother's family gets married, the gift money given by the sister and brother-in-law should not be too small. The amount should be based on local customs. Your wife can discuss it with your parents. If you are well-off, you can add more on this basis. That is your own intention, so that you will also be proud.
In addition, the specific amount of the gift money should be as good as possible. 6 and 8 are better. The number 4 should be skipped. It is best to be an even number, which means that good things come in pairs.
I recommend you to give the gift money + gift. It is best to give the gift before the wedding, and the value of the gift is up to you to choose. If you are well-off, you can give a large item, such as home appliances, but you should discuss it with your family to see what is missing to avoid buying too much. Or you can give more gift money and a gift of less value, which can be a decoration with auspicious meanings, or a set of festive bedding. Gifts should be practical, after all, they are given to your family, especially rural people who don't like vain things.