Kyose is the family name. Haruhito probably had a sister as the boy mentioned that "Grandpa Kyose" was from his mother's side. His father probably married into the family as it was commonly done in noble families with only a female heir. So the boy in the last chapter is Haruhito's nephew. ٩(๑❛ᴗ❛๑)۶
Well if the father married in then the boy's last name would have also been Kyose, so I think his last name was actually different. Haruhito Kyose was the soldier boy's full name.
The way the other boy said it implies Kyose was his mother's maiden name, so yes, Haruhito had a sister.
So the boy at the end was the dead soldier's nephew.
Wait grandpa kyose..??? This doesn't add up.. how could kyose have a child if he was a young boy, in a war, and in love with godo??