Here are our halachot (Jewish laws), and an additional halacha (Jewish law) as we now move to actual Erev Pesach (Passover eve). We have an inyan (matter) called Ta'anit Bechorot, and that is: any firstborn is supposed to fast. Why are they supposed to fast? Because we know that on this day, on Erev Pesach, the Bechor Mitzrayim, the Bechor of the Egyptians, were killed, and our bechorim (firstborns) are considered saved. Because of that, they need to fast.
However, we attempt to break that fast with the siyum (completion ceremony). The only question is: who is the firstborn? By the way, that does mean that until that siyum happens, there's no coffee and there's no eating before it; it's really a fast until that siyum takes place. So that's clear: the oldest from either parent is considered the firstborn. It does need to be the oldest of both parents. Let's say the parents from an early marriage, when they're married before they have a child from a previous marriage; it doesn't make a difference. The oldest from either parent is the firstborn.
Is it also the oldest females, or is it only the oldest males? That is a question; it's a debate. And Shulchan Aruch writes, Yesh omrim, some say yes: even oldest girls require a fast and therefore a siyum. And therefore we try to follow that and tell our oldest daughters and oldest girls that they, too, should be a part of the siyum one way or another, in order to have that fast broken for themselves as well. That is really how we follow, and that's the halacha: even the girls should make sure to try to be a part of the siyum to break that fast for themselves as well.
Have a great day. Kavua (fixed). Kavua.