@pikonyanko : Well, if you want a reeeeeally tragic setup for that.
The scale was actually super important so the dragon giving it to the girl was something of an ultimate sign of trust.
But the girl lose the scale somehow (stolen, trade it away without knowing, etc) so the dragon felt betrayed and killed her for it.
Although if we're sticking to comedy setting:
The scale was for proposal for marriage.
And the dragon didn't kill the girl, it took her back to its lair, the brother just collapsed before hearing the important conversations.
In fact if we're going full on comedy, it'll be that same sister who tell the priestess of the proposal scale thing for maximum impact.