I'm about to explode, what do you think I should honestly do?

Ok so my sister is 15 and her boyfriend is 18. While I think he's a clean cut guy I still haven't known him for but 5 or 6 months and me being her 18 year old brother, I probably shouldn't trust them when my dad's asleep in the other room and mom is at work and they're laying in her bed with her bedroom door shut. I'm not trying to be a snoop or to be dictating but I don't think the door should be closed like that while they're laying in her bed, it's not right. So I went and opened the door and of course she's like "really? you just did that." And I said that the door has to be opened and she says she doesn't want the dog coming in and I'm thinking, wow they should be fine they're both piled up in her bed what's the dog gonna do but I just left the door open and she gets up and closes it. I go back and open it again and I said "we're just gonna leave this open", not forceful but I knew it wouldn't go over well with her. So I told her to get a baby gate that we have in the spare room if she wants to keep the dog out, but otherwise it stays open and she said "no, I don't have to, get away" and so then I started to get forceful and said that the door is going to stay open no matter what and she screamed "go away, just get out of here" and not screaming back I just said "fine, but you're keeping it open" and she puched me in the stomach and went in the other room and slammed the door and she's been there ever since. My dad heard it, woke up, and said nothing...when I told him she's flipping out because she wants her door shut with her boyfriend in the other room in her bed and punched me in the stomach, he just said "I don't want anything to do with this go away, just go away". So I'm storming, I didn't yell or scream after that I just went back to my room trying to figure out what next and I'm p*ssed. I'm now questioning whether or not I should have even started this when I knew I was walking into a fire, but don't you think it's kinda wrong for her and him to be acting like that?! My question is that and what should I do at this point?