I think its a harlequin Romance ,I read it maybe 4 yrs ago or so. I dont have alot of details but this much I know. They were a younger couple and they split-up, he went into the air-force...? I think.  She found out she was pregnant and ended up giving the baby up for adoption to his brother and wife. He came back when the kid ( a boy) was 5ish. and considered the boy his nephew, they got together and close to their wedding day ,maybe a day or two ,he was in her house when he heard a message on an answering machine from her mother saying ' Did you tell him so & so is his son.

Does this sound familar to anyone?  I know I'm not great at describing it.

It might have been a silhouette romance .