Why can't I use "sin" instead of "hennes" here? It seems like in some contexts there wouldn't be any ambiguity about who is being talked about.
I believe it is because sin is for you have already identified who the person is.
For example: Hon ger sin son en bil när han är arton år gammal.
She gives her son a car when he is eighteen years old.
could you say "hennes son ar arton"?
"Hennes son är arton", yes :)
Aderton equals to arton