The resolution is that the girls wanted to meet their mother and found out she isn't a bad person.
The girls head back home to Brooklyn after learning a lot more about their mom. Sure, she's no Mrs. Brady, but she has her own stuff going for her. Delphine realizes things are way more complicated than what she thought before their visit, and while she might not become besties with her mom, she's one giant step closer to understanding the gal. Cecile and Delphine promise to keep in touch, which is way more of a resolution that we thought we'd get based on the beginning of the book.