Katherine was the daughter of Hamish, Emily and Honor Alexander-Harrington, the granddaughter of Alfred and Allison Chou Harrington, and the niece of James and Faith Harrington. Her older brother was Raoul Alexander-Harrington. (HH11)
After Honor Harrington had become pregnant from Hamish Alexander, Allison Chou Harrington suggested that Emily Alexander might also want to have a child, which of course had to be tubed due to Lady White Haven's physical limitations. Emily had never seriously considered having children after her accident because she didn't want to have a child that would inherit her genetic incompatibility with regen therapy, but Allison promised that this would not be the case.
Katherine was born two months after her older brother Raoul, shortly before Honor Harrington's return from the Battle of Lovat. Elizabeth III wanted to meet Honor immediately, but Honor angrily refused, saying the Queen of Manticore would have to "get in line and wait" behind Katherine and Emily. (HH11)