Is the paternal cousin who was breastfed by the mother considered a sister to the brother? Are her sisters considered sisters to the questioner and her brother? What are the limits of interaction and veiling in relation to the husband of the paternal cousin (sister through fosterage)?

If your female cousin nurses from your mother, she becomes your sister and a sister to all your brothers through fosterage. As for her brothers and sisters, they are not your siblings, and it is permissible for your brothers to marry her sisters, and for her brothers to marry you or your sisters. The husband of your female cousin is a stranger to you, and you must observe hijab in front of him, just as is the case with the husband of your full sister.