What had happened to Matthew and Rachel's two sons ? What does Kit vow to do after she learns this?

FRom the text:

"Mother has never told you much about our family, has she?" she went on. "You see, there was a boy their first child, two years older than I. I barely re- member him. We both caught some kind of fever. I got well, except for this leg, but he died."
"I didn't know," whispered Kit, stricken. "Poor Aunt Rachel!"
"There was another boy, after Judith," Mercy continued. "He lived only a week. Mother said it was the will of God, but sometimes I have wondered. He was very tiny, born early, but on the third day he had to be baptized. It was January and terribly cold. They said the bread froze on the plates at communion that Sunday. Father bundled him up and carried him to the Meeting House. He was so proud! Well, of course that was a long time ago, but after that Father changed. And it has been a struggle, trying to man- age without a son to help. That's all we meant, Kit."
Kit sat silent, her own bitterness forgotten. I will try harder to understand him, she vowed. But oh, poor Aunt Rachel, who had been always laughing!

Matthew and Rachel's sons died.

Kit vow to try harder to understand her uncle.