Although Jacob has taken the initiative to place a newspaper ad for a new wife and corresponds with Sarah via mail, he's still a bit shy when Sarah actually arrives:
Papa was quiet and shy with Sarah, and so was I. But Caleb talked to Sarah from morning until the light left the sky. (4.5)
He's quiet even around his own children, so it's no surprise he doesn't talk all that much when Sarah is around—especially since it's been a while since he's had a woman in the house. But that doesn't stop Jacob from getting to know Sarah. He shows her around the Midwest and even teaches her to drive a horse-drawn wagon so she doesn't feel too cooped up. He makes her feel comfortable and cared for, which is more important than anything that he could say.
Yes, Jacob Witting is a quiet man of few words, but he definitely cares for his family. His children know this without him having to say it, and Anna has memories of him being loving and affectionate with her mother. When Sarah comes, he shows he cares about her in a lot of different ways. For instance, he makes her a "hay dune" when she misses the sand dunes by the sea, and he puts his arm around her when a huge squall hits:
Papa said nothing. But he put his arm around her, and leaned over to rest his chin in her hair. I closed my eyes, suddenly remembering Mama and Papa standing that way, Mama smaller than Sarah, her hair fair against Papa's shoulder. When I opened my eyes again, it was Sarah standing there. Caleb looked at me and smiled and smiled and smiled until he could smile no more. (8.44)
It's clear to everyone involved that Jacob is a good man, and one who will protect his family no matter what. And he extends this same love and compassion to Sarah when she arrives, which probably makes her realize what a great husband he'll be.