Thank you for your comment, @bluedanube
It seems this inconsistency of their age can be seen when Sim-siblings are in the same age stage.
Whoever's elder is up to our interpretation.
I agree, and that might be what the devs. intended.
One problem is which expression should I use to describe them in Japanese, "兄妹" (older brother and younger sister) or "姉弟" (older sister and younger brother).