When I lived in my parents' house, I lived with my two-year-younger brother. He was kinda annoying like all little brothers, but he and I got along well enough. In fact, we got along well enough to share the bathroom in the morning before school during our shared high school years.
Every morning I'd wake up, eat, and get ready for school, without a shower. I'd always shower the night before so that I wasn't too rushed. My brother though needed to shower in the morning, so he got up a little earlier and ate before me. The way our scheduling worked out, we both used the bathroom at the same time: me to brush my teeth and him to shower. So I'd usually walk into the bathroom as my brother was in the shower (or sometimes he'd come in as I was brushing). Our shower had an opaque curtain so I couldn't see in when he was showering. If he was getting in or out of the shower when I was in the bathroom, he always covered/uncovered himself with a towel in the shower. So I never saw him naked.
Now for the masturbation part...
Being in high school, I of course knew that guys jacked off, and my brother had even admitted it to me. So when I heard funny noises coming from the shower when we first started our morning routines, I was pretty sure I knew what he was doing. At first I pretended to ignore it even though I was kinda grossed out by it. I masturbated too at that age, but not in the same room as my sibling so I thought it was really weird. But I let him be, figuring that boys will be boys and jack off in the shower. After a little while though I thought that maybe I should confront him. I told him one day that I knew he was jacking off and that it was a little weird. He told me he'd stop if I really wanted, but that he needs to jack off in the morning and that was the most convenient time to do it. He somehow persuaded me that it was okay for him to do it just a couple of feet and a curtain away from me every morning. So even after our talk he continued. Eventually I got used to it. We'd even talk by the end of the first year we did this. We'd just have a break in conversation when he orgasmed. Once he peeped his head out of the curtain and asked me to get more conditioner becasue he was out of lube and I did.
I guess there was nothing too too sexual, but there was definitely some tension every morning in our house.