Are the children of my maternal uncle from his second wife considered our siblings, given that my mother breastfed my maternal uncle and his brother, and my maternal uncle’s mother breastfed us?

Your two male cousins, the sons of your maternal and paternal aunts, who breastfed from your mother, have become brothers to you and your siblings through suckling. And your suckling, both you and your sister, from your maternal uncle's wife (your paternal aunt) makes you both children to her and to her husband, your maternal uncle. Thus, you both become siblings through suckling to anyone who breastfed from your paternal aunt's milk, or from her husband's milk. As for your siblings who did not breastfeed from your paternal aunt, they are not siblings to the aforementioned sons of your maternal uncle unless there is another cause for mahramiyyah (prohibition of marriage). It should be noted that for suckling to establish mahramiyyah, it must consist of five full, satisfying sucklings, according to the preponderant opinion.