I don't have a sister, but I grew up for a few years with my stepsister, who was older than me... when I was 5, she was a teenager.
I know she received enemas from my stepmother, my grandmother, and my aunt (a close friend of my stepmother), but always less frequently than me. I never watched while she received them because I wasn't allowed to enter the room. However, I knew very well what was happening and I wanted to watch.
Sometimes, while my stepmother or grandmother was giving me an enema, my stepsister would be there to assist. This happened especially when I was little (from 5 to 10/11 years old). As soon as she found out I was getting an enema, she would come in and chat about this and that, often commenting on the procedure. This started to bother me when I entered puberty, so my stepmother told her not to come in anymore.
It happened a few more times, but she would come in and leave without stopping.
The last time it happened, I was 13. I remember that my grandmother was giving me the enema that time, with a bulb syringe in the knee-chest position. She came in as soon as the procedure began... and stayed next to my grandmother, who didn't send her away. I asked her to leave, but they both downplayed it, saying that it had happened before. I remember my grandmother telling her, ‘This is the correct position for enemas’ and ‘It's okay for girls to see boys naked, but the opposite should never happen’.
She stayed for almost the entire procedure (after filling and emptying the bulb syringe 4-5 times) and only left when my grandmother pointed out my erection. I think my stepsister understood that it was time to leave.