Big sister

I nanny a 1 y/o M–F. His 8 y/o sister gets home from school 30 min to an hour before I leave, so I see her during that time as well.
The issue is that she can be very bossy with me. She’s constantly saying things like, “Just bring him here,” “Just do this,” or “Give me that.” She can also be a little rough with her brother sometimes. Throwing pillows at his face, taking his toys, or just overall aggravating him.

I usually have no problem telling her no or correcting her when I need to, but I also don’t want to be too harsh since I’m not actually paid to watch her. Her mom has told me that she is old enough to take care of herself, so I’m not sure how much I should be stepping in versus letting her do her own thing.
What would you do in this situation? How would you handle her being bossy and rough with her brother while still respecting the fact that she’s not technically in my care?