Is it forbidden to marry a man whose other wife is the maternal aunt of one's wife through rada'ah (i.e., the sister of the wife's mother through rada'ah), and must this marriage be annulled?

If your maternal cousin's daughter was nursed by your grandmother five times during the two-year period, then she has become your maternal aunt through suckling. It is not permissible for you to be married to her, and your marriage contract is invalid. You must separate from her immediately. However, if the number of sucklings was fewer than five, there is a difference of opinion regarding the establishment of prohibition through them, but the more cautious approach is to consider it as establishing prohibition. You must submit to Allah's decree and be patient, seeking Allah's help and striving against yourself to be patient, for "and whoever strives to be patient, Allah will make him patient."