A Promise to Keep. Juniper Bay, #8

Two strangers. One little girl. One impossible promise. Sadie Brooks lost her sister on a Tuesday. On Thursday, she found out she wasn't the only one. When her sister Claire and brother-in-law Tom die in a car accident, Sadie becomes guardian to her five-year-old niece Millie - the child she loves more than anything in the world. But Claire and Tom named two guardians. Sadie, and a man named Owen Reid - Tom's best friend, quiet and self-contained, a man Sadie has met exactly four times and knows almost nothing about.
The lawyer's advice is practical and devastating: a married couple has a far better chance in a custody case than two single strangers. So Sadie and Owen do the only thing two grieving people who love the same little girl can do. They get married. On paper. As an arrangement. Just until Millie is settled. Neither of them plans to feel anything. A deeply emotional marriage-of-convenience romance about two people brought together by grief, held together by a five-year-old who deserves the world, and slowly, undeniably, falling in love across the careful distance they both promised to keep.
For readers who love a slow-burn marriage of convenience with real emotional weight and a child who steals every scene. He kept his promises. She kept hers. Neither of them promised not to fall in love.