Wait does Hazuki have a brother? How come we never saw him?

That's not his brother

I know he's not, but his mother introduced Hazuki as her second son, so he must have an older brother right?

Hazuki is first son Naoto is second son (bc they are married) Hope this clears it up for you :)

Its soo sweet that she refers to Naoto as her son

Riiight, yeah its as son in law ヾ(☆▽☆)

Ohhh so that's why!!
That's so sweet of her but I was so confused