A sibling who knows your secrets and never uses them is a vault with your name on it.

Trust is only provable when betrayal would have been easy. A sibling has years of material on you, the kind that would land hardest in front of your parents, and every argument you have had was a chance to use it. Not using it is a decision repeated so often that it stops being visible.

Say it out loud once, since this is the sort of loyalty that never gets mentioned. Then hold the same line with what you know about them, including when it would win an argument. If you have used something once, say so plainly, because that is repairable and ignoring it is not.