Is it permissible to marry the daughters of the sister of the brother by suckling, and is it permissible for my brothers to marry them, and is it permissible for him to marry my sister?

The one who suckled from your mother is your brother and a brother to all your mother's sons and daughters through suckling. It is not permissible for him to marry one of your sisters. As for his sisters who did not suckle with you, there is no prohibitory relationship between them and you or your brothers, and it is permissible for you to marry them.