Help me with my odd ball family? Please!

I have a question for the male population, preferably the married ones. I live in a apartment with my sister,r her husband and their three adorable children, and my mom. It's not as cramped as it sounds. Although some days it does feel that way. My cousin whom is 17 is currently staying with us. Since she has gotten here. She and my brother in law has spent every waking moment with each other. Even when she sleeps in our living room he is there with a blanket sleeping on the floor. Sometime when I come out my room for a midnight stroll to the bathroom I can find them cuddled up on the couch. During Thanksgiving dinner when a small amount of our family was present they were doing it yet again. This time his wife, mother in law, and grandmother in law were present. In my head I'm thinking, are these two idiots aware of their surroundings. I mean come on now. It your cousin's husband and the man isn't even that attractive. Which makes me wonder what my sister saw in him in the first place. The two of them often go out together, just the two of them. At first I just thought they were a just super friendly. Of course this is a cute little naive 18 year old me. I tend to try and see the best in every person who I meet. Even Charles Manson has some good in him, OK maybe I'm a little overly optimistic. But something just rung me as odd in the past few days. When he was in bathroom, so was she. When I came out the shower a couple days ago, I found her lying in his bed with her spaghetti strap for her tank, off. And just last night he gave him an ice cube from her mouth directly into his. It was the most disgusting thing. It is killing me that my sister sees these things and says absolutely nothing. Once current ex said he saw them laying on top each other and the both of them were asleep.(He stopped by often when I wasn't even here, his mom and my mom are pretty close) But to get on with t he story of my disrespectful relative I feel that I should just ask her about it but the red light goes off in my head. Can anyone explain their odd behavior to me?