Thought Experiment
So let's say you start dating someone who has a (small) child. Your significant other is a single parent to this child; the child has no other active parental figure.
You marry this person, but do not legally adopt the child (this is important). In time, you and your spouse have two more children. Your family family (you, your spouse, stepchild, and two children) have all holidays with your extended family because your spouse doesn't have one and consequently neither does your stepchild.
For one reason or another, it doesn't work out between you and your spouse, and after 5-10 years of marriage, you divorce. Your spouse, for whatever reason, gets main custody of your two children; your former stepchild is also still a minor living at home with them.
Now: when you take your kids to your family's traditional holidays, do you take their half-sibling too?
You marry this person, but do not legally adopt the child (this is important). In time, you and your spouse have two more children. Your family family (you, your spouse, stepchild, and two children) have all holidays with your extended family because your spouse doesn't have one and consequently neither does your stepchild.
For one reason or another, it doesn't work out between you and your spouse, and after 5-10 years of marriage, you divorce. Your spouse, for whatever reason, gets main custody of your two children; your former stepchild is also still a minor living at home with them.
Now: when you take your kids to your family's traditional holidays, do you take their half-sibling too?