Being the oldest sibling is a leadership job nobody applied for. Do it kindly.

Nobody interviews for that position, and it arrives with real duties anyway. You get watched, copied and measured against by people who did not choose the arrangement either, and the two usual responses are to take the authority too seriously or to resent having it. Neither works well. The role is real whether or not it is fair.

Kindness is the part you get to decide. Explain things rather than just issuing them, let your siblings be bad at something without commentary, and apologise properly when you get it wrong. You are also allowed to tell a parent, out loud, that you are tired of it.