Please answer asap, I feel sick that my younger sisters were the same room. Read and please answer. What does this mean, what is going on?

My parents have sex every morning, I am not spying on them they just make it very obvious. anyway for the last two weeks me and my sisters have been sleeping in our parents room, I sleep on the floor and my sisters sleep on the bed with my parents ( my sisters are 8 and 6) now last Sunday morning I woke up very sick feeling and my mom got up to get the house phone because my grandfather was calling, she sounded a little annoyed that I was up and she went down stairs, after a little I went down because I really wasn't feeling right, she stood with me for a little and then she said her head hurts she has to lay down, so after a little I planned on going back up to lay down and when I went inside my pillow plus blanket was on the couch and we I went upstairs the door was closed and I didn't even bother to try to open it. Now I think they had sex with my sisters in the room sleeping, or they were doing something because when that door is close you know they're doing it. I feel sick, the other weekend before this happend, (which was a Sunday as well), she kinda did the same thing and once I went back upstairs the door was a little slightly close but they were laying down together and that was it. I don't mind that out all BUT what brother me was this last Sunday. I feel bad kinda for my sisters, it's just werid how she left me sick not feeling good plus did whatever with my sisters in the same bed as them. What does this all mean? What do I do? I just feel bad for my sisters