Juliet writes to Sidney and asks him to come back to Guernsey to give her away at her wedding on Saturday; she's marrying Dawsey. Juliet says that her whole life has come into being in the last day, and she doesn't even care if it's unseemly to get married so quickly. She's excited to discover how interesting marriage is and suggests she'll write a book about it. Dawsey arrives to interrupt Juliet's letter writing, and Juliet says he's no longer shy. She includes a postscript saying that even Adelaide is happy they're getting married and formalizing their relationship.
The fact that Dawsey isn't shy anymore after getting engaged speaks finally to the power of creating a family: it makes Dawsey better able to communicate and interact with everyone around him. By asking Sidney to give Juliet away, she asks him to assume a role often held by blood family, thereby formalizing Sidney's inclusion in Juliet's chosen family.