Its really awkward between me and my sisters boyfriend...?!

Wow..they are old enough to know better! I would tell your sister that you were uncomfortable with seeing him nude and being 13 and not YOUR boyfriend!.I will calm down now..I am angry for you.. I would like to ***** your sister out for you!
Your sister is a c#%t and her boyfriend is an asshole pervert..He probably walked in on you on purpose and I am suspicious of their behavior towards you.. WHERE are your parents?
I am serious..this is not good...tell someone like your parents! who is in charge of this household?
You need to tell someone to tell your sister because clearly she has NO common sense..
get back to me..I am concerned about your safety with people treating you like this..please answer back.
I am worried about this guy being nude and your sister okay with it.. It is not right and there is something wrong with the 2 of them.. be careful, lock your door to the bathroom and your room if you can..
you are trusting your gut...go tell someone in charge..this is not accidental behavior on their parts!
Please tell someone!

Sounds to me like this guy is a pervert and a possible pedophile. Just avoid him or tell your sister that you're not comfortable with him being there. If that doesn't work, tell her about the incident about him being on the bed naked knowing that you were in the room. If your sister doesn't care, tell your parents so your dad can kick his a**.

First of all you shouldn't have to avoid him in your own house. He's got to go. You NEED to tell someone, your parents because obviously your sister isn't going to be of any help. If you feel unsafe around him it's probably because he is. Tell someone and stop things before they get harder to handle. Good luck.