Like, notice how upset she was about him flashing himself at her?
I think that possibly they may have had sex at some point or another and she doesn't want anyone to know yet he keeps going on about it because he thinks its funny. She also said last night something along the lines of "he knows things about me" before changing it to "or what he thinks he knows about me"..
and then in the diary room she was saying "My family watches this its so disrespectful" and crying. This seems almost to me like she was trying to signal to big brother that he was trying to reveal something that she didnt want her parents to know, and she was talking about Marc flashing at this point which makes me feel it is somewhat relevant to that speech.
Do you guys think this is possible or am I talking ****?
