It's been a while since my last family trip, and I'm going to take a bath with my sister! I saw my sister naked (with big breasts) for the first time in a long time, and it was too stimulating for me, so I got an instant erection! But I couldn't let my sister find out, so I tried desperately to hide it... but she found out right away... Just when I was preparing myself for her to scold me, she put her hand on my crotch! Why? My sister came to me in a slightly panicked state!