I immediately thought of one of Nora Roberts' Irish (or as some internet wit rightly put it, "fetish-Irish") trilogies, the Born in series. The first two books are the romances for the sisters Maggie and Brianna, but also feature their complex relationship, which is healed in the first book. The third book brings in an adult half-sister hitherto unknown, which does NOT go well initially. The growing bond of the sisters is almost as important as Shannon's romance.