Is it normal for a teenager to...?

act disrespectful to her family when her boyfriend is around? My younger sister is 17, and whenever her boyfriend is over or around, she talk to us with a completely different tone, like she doesn't care about what we are saying at all. She will ignore a lot of what we are saying and give her boyfriend her undivided attention. Also, whenever he is over, he never eats what our mom cooks for dinner, and because of this, my sister doesn't either. Even if it is a dish she really likes. They are always engaging in PDA and sometimes they will stay in the room that she and I share for hours with the door closed. And my mom lets this happen. When my dad is home they can't close the door, but it might as well be since its only cracked and no one bothers them. She seems to have absolutely no regard for anyone else's feelings except for hers and her bf's when he is around. I admit that I haven't spoke my mind much about her behavior, but it seems like that would be my parents' job? I dunno. I guess this is normal for immature teenagers, but what do you all think? Are your siblings this way? Thanks for reading!