Well if i understood you well woman A and woman B breastfed each others elder son. So this son's are both considered as son's of each of the women.
But this doesn't apply for their brothers or sisters. And therefore neither the daughter of woman A nor the 2nd son of woman B are by any means considered as foster sisters and they are therefore foreigners to each other and could get married.
Ibn Qudamah (may own translation,take it with care!) said: The child
who has been breastfed is a mahram for his Foster mother and Foster
siblings. And the same situation applies for his children and
children's children etc. but it doesn't apply to his siblings or his
parents or aunts (both maternal and paternal - this also applies for
aunts and uncles in the follwoing) and uncles. Also the foster mother
could marry the father of her foster child or his brother or uncles.
And her husband could also marry the mother of his wife's foster son,
or his sister or aunts. And the children of the foster mother and her
husband -who are not foster siblings to the child- could marry the
siblings of the foster child. Imam Ahmad said: A man can marry the
sisters of his foster sister as there is neither a lineage nor
breastfeeding between them. The breastfeeding only occurred between
his foster mother and his foster sister.
Here are two references in Arabic 1 & 2
And Allah knows best!