My sister's fiance has another family, she doesn't know

My sister is soon getting married to a man she is madly in love with. I have never seen her happier in life, but recently, I discovered that her soon- to- be husband, who I will call Jack, has fathered two children with my colleague.

I have spoken to my colleague without revealing my connection to the couple (my sister and Jack) and she has even showed me pictures of her happy family - Jack, herself and their two children.

From our conversation, I could tell that my colleague has no idea that the father of her children is getting married to another woman. My sister also has no idea about Jack's other family.

I talked to one of Jack's cousins and she admitted to knowing about his two children. This cousin said Jack had told them to keep quiet about it and that he would tell my sister after the wedding.

Should I confront him about it? Should I tell my sister? I know that if I were in her place, I would want to know. Please advise.