Siblings usually are very jealous of each other. In my opinion it would be only fit to negotiate an action that would recompense the younger kid but only after it settles down (screaming and whining is not the way to gain anything).
You were in a car and a whining child can seriously impair your attention. Stop of you have the time and give the kid some time to settle down. Also asking the 17y old to go out of reach of hearing would make things easier as she constantly reminded the younger one about the unfairness.
Settle a firm requirement the kid has to follow before you even start listening.
After she settles down negotiate if she followed your requirements to the letter. If not drop the subject and carry on going back home.
You can teach kids about fairness early on and it goes very well with establishing a position of control. If you bend once to a kids fit you are in trouble because the next time it's possible the kid will try to leverage your weakness for screaming to his/hers advantage.
Stories about life and fairness are hard to relate to for kids. Give simple examples. But first make sure the kid is not trashing about because it will not listen.
Talking about feelings can be useful in a rage fit. If you make her tell what she feels (e.g. you being unfair makes her furious) and what you feel (bad because you forgot about her) it should make her understand you more and vice-versa.
If after 3 min. you are unable to gain any control the cause is lost. Ignore until she calms down and then you can again try the feeling approach. Once that is settled dissect the situation and point the overreaction on her part. If this turns out ok promise her that next time you will remember about her but do not give gratification immediately as it will still boil down to her rage giving her profit.
When it comes to your kids you will have the subject pop up probably even before the kid will be able to speak and once he/she goes to school it will pop up daily.
Always be firm in your stance and clear about positive and negative consequences. This should in itself make you a great example of fairness :).