Kino and Juana's relationship demonstrates the power of love to overcome the most serious of adversities. Despite their extreme poverty, theirs is a close relationship. They may not have much in life, but they have each other, as well as their son, Coyotito.
However, Juana is different to Kino in that she understands fairly early on that the valuable pearl will bring them nothing but bad luck. Kino doesn't understand this until it's too late, and until then, he inadvertently puts his family in danger by hanging onto the pearl in the forlorn hope that he will somehow get a fair price for it. As a result, Kino and his family are hunted down like animals by unscrupulous pearl traders and their hired thugs, with tragic results.
At no point, however, does Kino and Juana's relationship fundamentally change. If anything, their numerous bad experiences bring them closer together. It's not an ideal situation, to be sure, but it's often in cases of the greatest hardship and adversity that the underlying strength of a relationship truly emerges. And so it is in this case. By the end of the story, Kino and Juana have lost the pearl as well as their son, but they still have each other.