How is $A.in$ an inherited attributed in $2nd$ production in $(1) S \rightarrow A_1A_2\{ A_1.in := 1; A_2.in := 2 \}(2) A\rightarrow a\{print(A.in)\}$
But in the above excerpt, $A.in$ for the nodes $A$ of the parse is not defined in terms of attributes at the parent of siblings.

Yes, it is. The parent of $A\to a$ is $S \to A_1 A_2$. $S$ has two children, both of which are instances of $A$. And the rule for  $S \to A_1 A_2$ assigns values to the inherited attributes of both children.
That's precisely what an inherited attribute is. Inherited attributes (which are inherited from the parents in the parse tree) can be used in the synthesized rules for those non-terminals, which is what is happening in the rule for $A$.