The child who teaches a sibling may know the subject in ways a worksheet never asks for.

Teaching a sibling requires knowing where they will get stuck, how to say it another way, and when to stop. That is a more complete grasp of the material than any worksheet examines, and the child doing it usually gets no credit for it at all. Explaining is the harder half of understanding.

If you want to know whether you understand something, teach it to someone who does not. Notice where you have to reach for a second explanation, because that is where your own grasp is thin. And notice the children who do this, since their capability is invisible to assessment.