Sloppy people and Liars (help!)

[phoenix_suzaku18]

I currently live at home with my mother and younger sister. Last July 
we moved in with my mom's boyfriend, Tom, and his two children.
I am 20, and going part-time to college. My sis is 19, and working. 
Tom's children are Shane 18, and Andrea 15. I have huge problems 
dealing with their behavior. Shane says up all night. His waking 
hours are between 4:30pm-8:30am. Which is fine for him, because he 
works 4 days a week from 5 until 11pm. He has not graduated from high 
school, nor does he have a drivers license. My sister or I end up 
driving him to work. Anyway, to the problems... Shane's room is an 
experiment in SLOTH. Cigatrette packs and Coke cans litter every 
surface, including the bed. Since moving in, he has not changed his 
sheets. He showers once every 3 days, despite being a fry cook. I 
pick up litter around the house after him and his sister; banana 
peels, used tissues, cigarette butts (on the floor! we have ash 
trays!!), you name it. Also, I've caught him lying several times. 
Examples: Just 5 days ago my mom had purchased a carton of Tom's 
favorite cookies for him, and hid them in our pantry. They had yet to 
be opened. Yesterday, I found the carton w/only 4 cookies (out of 33) 
left next to the computer where he spends each night. I regularly 
bake cookies- a variety too- and Shane had plenty of food options. 
Last night as I was baking more, Shane inquired "what kind are 
those?". I told him and said, "You could have one if you like". He 
replied "Thanks, but it better be just one, otherwise I'll carbo-load 
and get fat."                                                        
I said, "Like last night, eh?"             Shane, "Yeah, I ate like 
half a box of cookies."     Me, "What do you mean? there were only 4 
left?"  denial denial denial GRRRRRRRRRR...etc...
.....His dad, Tom, was missing his last 2 packs of cigarettes last 
week. My sister and I do not smoke. Tom asked everyone where they had 
gone. No one fessed up. My mom did know who, but for fear of 
embarassing Shane did not say anything in front of everyone. The day 
before he had told my mom that he was out of cigarettes, and would 
have to pay his dad back, because he had taken 2 
packs.                                                               
Why would someone lie like this? I have been raised to be very 
honest, and can't comprehend why anyone would go out of their way to 
lie like that. I do understand that these examples are not severe, 
only severely annoying. Plus his lack of personal hygiene makes 
everything bad about him seem 
worse.                                                              
Does anyone have any advice?