Are you the youngest? If so the 'kids' are no longer kids and if they were not happy or in love one would have divorced the other by now!
Kids learn from their parents. When a child sees their parent hitting the other or doing drugs or have sex wiht whomever, then they learn that it is okay to act that way. I am speaking of a majority not every child.
I never saw my mom get hit or even hear my dad raise his voice to her. They seemed happy. Yet he still had an affair(s). I guess that is what started me into being a little fast and loose. That is until it happened to me!
At your age you have learned from them and YOUR peers. You know right from wrong. Socially/Sexually acceptable from not. You are at an age and maturity level that you can make and act upon your own decisions.
Have nothing to fear. If you object to a behavior or know that something is against your own personal morals code then the line is drawn in the sand.
A child starts learning sexual behavior and moral before primary school, so think about how early they start learning other morals and habits and reactions from their parents.
As far back as I can remember my father would slip his finger through his key ring and spin the keys around until they slapped into the palm of his hand while he said, "Alright, come on, let's go!" I never realized that I too did that. Not until my wife noticed it and said something. I moved out and have been on my own and in another state since I was 15, some call it running away, I called it being overly independent.
Also, nudity. It is learned (especially here in America) that nudity and a naked male/female is sexual, as is breasts and penises and asses. That nudity is not social. That is it dirty, perverted and wrong. We teach that from when the child starts running around pulling off their diaper. We make sex and nudity so taboo that there is a high rate of sex in minors, teen pregnancies, high teen STD rates and teens that have had multiple sexual partners at one time. That it is okay for girls to be gay/bi-sexual but not men. Go to another country and watch TV or walk the 'public' beaches that are 'clothing optional' then look at their sexual related crime rates (rape, teen pregnancies, STD rates...etc.) and what do you see, they for the most part are lower then here! Can't even teach them about sex or B.C. in school because it is inappropriate or because it is against someone's RELIGION.
Point of this all...kids are like sponges. They soak everything up weather you want them to or not! Raise them right, know that sh*t happens and make sure their moral compass points north!