Jethro is Ellen's youngest, and I think that in detaining him so long before sending him off to Shad's, she is trying to hold onto him, reluctant to let him go off by himself the mile through the snow. Although Jethro is capable, I think Ellen is feeling sentimental, because all her other sons have gone off to fight in the war. The Creightons have just received a letter from Tom, telling them that he and his brother Eb are all right, but outlining the hardships and horrors they have experienced and witnessed. All of her other sons are gone and in the line of danger, so it is completely understandable that Ellen would want to keep Jethro safe by her side for awhile longer.
Despite the fact that Ellen might want to hold on to her youngest, she knows that he is hungry for company now that his brothers are gone. She sends Jethro to Shad's also so that Shad can read the letter from Tom. News from the front is infrequent and much valued, and the family knows that Shad will be anxious to read the letter. Jethro is also supposed to invite Shad over to the house for dinner. The family enjoys Shad's company, and Ellen knows that Jenny especially will be delighted to see him again before he leaves for the war himself.
Shad welcomes Jethro happily when the young boy arrives at his cabin. He helps him out of his coat and seats him in the armchair in front of the fire, and the two talk companionably about the family and about the war. Shad expresses his frustration that Mr. Creighton will not let him marry Jenny before he goes away to fight, and then explains to Jethro exactly what is happening in the war, and why the conflict will not be resolved quickly or easily. Jethro is very honored to be treated like an adult by his teacher, and the two share a delicious meal, and then Jethro sings for awhile while Shad plays the guitar. Jethro then goes to bed, while Shad stays awake to do some writing. The two will go back to the Creighton house together the next day (Chapter 4).