1,991. Warning One's Betrothed or Yevama

Hilchos Sotah 2:4

When a minor girl is married off by her father, she becomes prohibited to her husband if she commits adultery. She is therefore warned not to cause her to drink the bitter water but to forfeit the value of her kesubah, as has been discussed. A minor girl who is able to annul her marriage through refusal (miun) is not warned because she has no interest in becoming prohibited to her husband. Such a girl is not prohibited to her husband even if he is a kohein (since this marriage is not binding under Biblical law).

Hilchos Sotah 2:5

If a man issues a warning to a woman he has betrothed or to his yevama, and she was secluded with the object of her husband’s jealousy after their marriage was consummated, then she must drink the bitter water like any other wife. If a man issued a warning to his wife and she was secluded with that man before her husband was intimate with her, then she doesn’t drink the bitter water. Rather, she is divorced without being paid the value of her kesubah and she is prohibited  to her husband forever as per Numbers 5:20, “Another man has been with you besides your husband.” We see from this that the sotah ceremony requires that intimacy with the husband must have preceded intimacy with the man who is the object of his jealousy.