Am I allowed to marry my cousin, my maternal aunt's son, whose older brother was nursed by my mother along with my older sister?

If there is no proven suckling relationship between you and your maternal cousin (son of your maternal aunt), then there is no objection to marrying him. The fact that his brother was suckled by your mother has no bearing on this. The brother who was suckled by your mother became your brother through suckling and a brother to your sisters. As for his brother who was not suckled, he remains a maternal cousin whom it is permissible to marry. The jurists have pointed out that suckling is specific to the suckling infant himself, not extending to his brothers and sisters. Furthermore, the prohibition extends to the descendants of the suckled child, no matter how far down, but does not extend to those of his same generation or those older than him.