Is it permissible for a man to marry his first ex-wife if it becomes clear that her father is the foster-brother of his second ex-wife, knowing that he consummated the marriage with the second (ex-wife) before the 'iddah of the first (ex-wife) expired, and is the second (ex-wife) considered the paternal foster-aunt of the first (ex-wife)?

It is not permissible to combine a woman and her paternal aunt (father's sister) through fosterage, due to the Hadith: "What is forbidden by lineage is forbidden by fosterage." And if it is forbidden to combine two women, then it is not permissible to marry the other until the first one's waiting period ('iddah) ends, because a woman observing her 'iddah is considered as still being in the status of a wife.