Chapter 4
Tobirama's mom (not Hashirama's) had a kid before having Tobirama. The kid, Mei, is 1 year older than him, August 18th, while Tobirama's is February 19th.
Tobirama and Itama were supposed to be born on the 18th, but there were some complications with the birth, and Itama almost died. Their doctor was promptly never seen again.
Mei and Tobirama met only a handful of times before the Hatake's joined Konoha. Once, when they were 6 and 7, again when they were 8 and 9, 13 and 14, and lastly 19 and 20.
Mei kept their distance as they believed that Tobirama didn't like them and didn't want them around. While Tobirama thought they were fine and could only form an opinion on what others said about them. They only talked for more than 4 minutes during the last two times Tobirama visited, but only for 30 to an hour.
Mei's wolf gets along great with Tobirama's, and that's where most of Tobirama's opinion comes from.
When a Hatake is born there's usually a wolf that is also born with them. But in cases of half Hatake's and adoption they get assigned a wolf or share their parents wolf.
Mei is Kakashi's great grandmother, her daughter, Hotaro, is Sakumo's mother and died when she was 26.
In Hatake residence there are chests full of their history, but can only be open by the head and heir. Sakumo wanted to wait when Kakashi was old before teaching him about their family and didn't tell him much even when it was only them left.
There are furs and body paint and tattoo designs left in there.
Oh and for the Siblings chap -
Red, orange, and black are the colors the Iron pack
Green, brown, and dark gray for the Earth pack
Yellow, purple and white for the Lightning pack
Blue, pink, and light gray for the Mist pack
^^ colors for the body paint and tattoo's ^^
Only the Iron pack went by Hatake later, the others chose different names.