Protecting your younger sibling is loyal. Controlling who they hang out with isn’t.

There is a line between shielding somebody and running them. Watching out for a younger sibling means being available, being honest about what you know, and being the person they can call when something goes wrong. Deciding who they are allowed to see takes that away, and the usual result is that they stop telling you things. Then you are protecting nothing, because you no longer know anything.

Aim to stay informed rather than in charge. Say what worries you once, plainly, and then keep being the easy person to tell the truth to. If something is genuinely dangerous, that is when you bring in a parent.