Ohh, there’s a saying—“What your mother wishes for you, no one else can wish for you” — it’s used in the sense that a mother’s thoughts are so strong that even enemies can’t do as much harm to you as your mother can, haha.
So it means you were something like a backup, and it’s “set” that the first child might potentially die. For you, this can mean that you can’t really anchor yourself in one place—for example, someone might have one hobby their whole life, but in your case this probably doesn’t exist, because a root may not be “set,” or the family matrix is waiting for the first child to pass away so your life can start moving. It’s important to clear this for both you and your brother/sister, because with such thoughts, incidents can happen in families and someone can suddenly die in a car accident, a sudden illness, etc. Work on the first child as well, not just on yourself, to release this.
I would use the genogram to connect with my mother even before the first child and change her reality and thoughts, and then also for the second one (you). It’s good to hear them first so you know exactly what they are, and then suggest replacing them with other thoughts and a different perspective. Most of the time people accept it, because they actually love their children, but they also carry a lot of “nonsense” from the ancestral system.