DBNK-014 - While being washed by my older sister in the bathroom… could it lead to manual stimulation for 4 hours?

I was taking a shower when my older sister said, 'Let's go in together sometimes! Hehe.' Thinking it was a joke, I said 'Okay～,' and then my completely naked sister actually came into the bathroom… 'I'll wash your back.' A sudden all-nude washing service from my sister! My younger brother couldn't hide his erection and gets noticed by his sister. What does my sister do when she sees my younger brother's strong male organ!?