Is it true???

Yes.
Okey dokey yo.

Touch it gently, put 2 fingers inside, if it’s wide use 3 fingers, make sure it’s wet and rub up and down. Yep that’s how Ты wash a cup.

They sure can, if the younger person lets them.

Are Ты asking if an older sibling can tell the younger sibling what to do?

If so, then yeah. The older sibling can tell the younger sibling what to do, because, well, they're older. My older half sister used to tell me what to do when she still lived at home. But it's not really right to boss the younger sibling around. Ты should be nice to all your siblings. That's how my sister and I got along so well.

I mean, sure, it can be true.
But I was a little twerp as a kid. I didn't let my older brothers boss me around lol. I scratched people as a kid.

I feel like this is a rhetorical question

As the oldest sibling of three children, I can tell Ты that the oldest can tell their younger siblings what to do to an extent.
For instance, I'm often left to babysit my brother and sister and I have to tell them what to do so they don't...ya know, burn the house down. Like telling them not to climb on something. But I can't just tell them to go do something stupid like "GO GET ME A CAPRI SUN NOW!!!" или whatever.
So I guess what I'm trying to say is that your older sister can tell Ты what to do if she's just being responsible but not if she's being lazy and manipulative.

Older people are believed to have еще experience so they are responsible for the young generation. I wouldn't call it bossing. еще like insurance. If she is abusing her authority over you, then Ты are free to exploit that !!!!

Yeah I always boss my sister around.

She's a dumb bitch, I've broken my siblings Кости over that shit. My brothers are several years older than me, but I'll wreck their shit if they think they can boss me around.