There is another thing: Shev is a guy, and he is dealing with a
girl. There is also the gender issue and in this case their
relationship with each other is not that strong as siblings, so
there may be some difficulty.
What I would most likely do in his case:
IF I was found to be doing something she know of and she reported
to both parents and both parents gave black faces, I would give a
smile and tell them, "Wah... Bao Qing Tian lai liao!!!"
If they are rather serious, then I would STILL give a smile and
tell them, "Now, tell me how did I upset you." Listen to
everything, be it they explain it angrily or whatever, listen. Then
tell them what would they want me to do or NOT do. Then I do as
they say... for the moment. 
On the sis side, I will go to her, sit down and observe her. I make
sure she talk 1st so she can tell me what's her reaction then I
reply appropriately.
She may say, "You are sickening leh! Why are you in my room or
disturbing me?"
I will smile and reply, "Well, you were in my room so now I am in
yours." Or "Orh." Then go out. No confrontation with her at all
cost. Slowly go into "her life" then wait for right opportunity and
then have that really good talk with her.
