The first poem to consider for a daughter-in-law is "Snapshots of a Daughter-in-Law" by Adrienne Rich. While not a "hearts and flowers" poem, it nonetheless presents the image of a girl who begins her married life as a servant of sorts and evolves into a strong, independent woman.
For more emotional fare, traditional offerings such as 1st Corinthians 13:4-13 or Elizabeth Barrett Browning's "How Do I Love Thee (Sonnet 43)" are worthwhile options. Both of those poems focus on the importance and gift of love, whether romantic or familial. Another poem to consider is "By the Seaside" by William Wordsworth, who uses nature as a metaphor for the soothing comfort of love in response to the challenges of life.