Bundle of Joy by Robin Lee Hatcher

Alicia Harris says she's happily married and expecting a baby—but only the last part is true. She can't bear to disappoint her grandfather by telling him she's two months away from single motherhood. Then Grandpa Roger drops in unexpectedly to spend the holidays with Alicia and her husband, and Alicia needs to find a fill-in—fast. Childhood friend Joe Palmero fits the bill, and is willing to play along. Still, the longer they spend playing their parts, the closer Alicia and Joe come to discovering what love, faith and marriage truly mean.