I wish I knew. Logically, it should follow in the order of creation. For instance I make a dad, the mom, and from them generate their children, in the order I see in my own head. But, sometimes, the mom will be first, then the Dad, and sometimes the second child will precede the 'older' one. Face/palm. What I end up doing is aging my Sims in their 'birth order' and often times I need to make the mom a YA if the dad is an Adult. That way at least the parents follow their assigned placement. Sometimes I think it's better to just have the children born in-game. Ooffdda.
