Temporary Dad by Lynne Marshall

Hunter Phillips has to look after his baby niece for a month and is completely out of his depth! As if that weren’t enough, he also finds himself working with his ex-fiancée, Amanda – the woman who left him when he told her he never wanted children.
Amanda Dunlap is on a mission to help people stay well. She’s created curriculum for The Mending Hearts Club and is ready to test out her program on the first twenty candidates. When her mentor must back out last minute, she depends on Mercy, Inc. Admin to send a substitute. Only because she is out to prove herself to hospital administration, her parents, and the world, does she accept the man who broke her heart as the replacement.
Hunter is still fiercely attracted to Amanda, and she’s amazing with baby Sophie. As they pretend to play at happy family, Hunter begins to realize that winning Amanda Back and having a family for real is what he wants most of all. All he has to do is convince Amanda.