This is what we do

Dogfather because we all love padfoot PART I

After years of me thinking this whole scene is really sweet I never really thought about it but look:

He's not your son said Sirius quietly
He's as good as said mrs Weasley fiercely who else has he got
He's got me !!
Yes said mrs Weasley, her lip curling the thing is it's been rather hard for you to look after him while you've been locked up in Azkaban hasn't it ?
~from the book

Think about this for a moment think about how hard it must've been for him to say those words
"He's not your son"

Like no no no Molly this is my Harry my kid James and Lily's son from his appearance right down to the way he writes the alphabet and protects his enemies I've known the kid literally since he was born I know what James and Lily wanted for him they'd want him to know what he's getting into they'd want him to know that we trust him, and lily would skin me alive if I let her son face the chosen one scenario without knowing what it means James and lily potter gave their lives as a result of this prophecy and you're telling me they would wanna keep in the dark? He is my godson Molly I would do anything to keep him safe I am the one Harry wrote to nearly every day for months and I know what he needs I know what happened in that damn graveyard and I know what Harry's been through and I know what his parents would want us to do
HES. NOT. YOUR. SON.

"He's not your son" said Sirius quietly

Sirius is canonically the sort of person who would get increasingly louder and angrier over the course of an argument. But no. Molly want harry to be her child. Her child and all he can think of is lily. Her grit. Her principles. The way she's have laid the truth out before Harry and then have taken him to a quidditch game or something

He never gets to say any of that. There's molly's below-the-belt Azkaban taunt and Sirius just retreats into his guilt about not actually being there for Harry ... not being able to protect him last year ... not keeping James and lily safe...