My boyfriend's sister and my siste are basically obsessed with our relationship?

My boyfriend and I have been dating for 3 1/2 years and I live with him and his family until we start college in the fall. This past weekend we went down to Delaware where their family has a two trailers. It was my boyfriend's sisters's birthday so she had her two best friends one happens to be my sister. They slept in one trailer and my boyfriends brother and parent and us slept in the other. We went to a mini beach out in the bay. And they were like Awww and ur too cute whenever we were near eachother basically. This morning we woke up to them taking pictures of us sleeping in bed. They just kept taking pictures o us whether we were snuggling, holding hands, hugging. It was kinda creepy I would ask them to stop and they just giggle. They are 14 & 15 years old. Why do you think they would do this? because I really just don't understand.