Must I divorce my wife, or can the marriage remain, after it became clear to me that my wife's mother breastfed me naturally when I was one year old, knowing that I was unaware of this before the marriage?

Whoever is suckled by a woman five known sucklings within the two years [of infancy] becomes her son and a brother to all her children. A prohibitive suckling is one that is a separate meal from another.

If you were suckled five times by your wife's mother, and this is confirmed by the testimony of a trustworthy man or woman, then your wife is your suckling sister, and she is not permissible for you. You must separate without a divorce, as the marriage is void, and divorce is a branch of marriage. If there is a child between you, he is attributed to you due to the marriage contract coupled with ignorance [of the impediment].

However, if the suckling was less than five sucklings, or if there is doubt about their number, then the prohibition is not established, and the marriage remains valid, because the default is non-prohibition, and certainty is not removed by doubt.