i think my sister is depressed?!

my sister is 13 and she has always been the quiet type. but over the past year she has become more shy and down and always locks herself in her room all day. she's the type of girl who doesn't share her feelings and stuff like that, but i can tell she's down.

we're a really close family and stuff, and we all thought it could be hormones because she's at that age. but i'm not convinced it is. she's told me that she was depressed in the past and i've been up all night before with her crying on my shoulder before. i'm really starting to worry about her.

she's wants to be a director when she's older and she would be a great one. she's always making little movies on her camera. but lately hasn't been making any.

she's also been acting very stupid lately. she's been steeling our mum and dad's alcohol. but recently she stole her friends sister's cigarettes and smoked them. i of course found out and confronted her about it and said how wrong it is that she's stolen and that she's only 13! when i asked her why she said she couldn't tell me and that i wouldn't understand. i asked if it was peer pressure and she said no and that it was her idea. i asked her if it was because she was curious, she said no.

i then asked her why she's been acting like she has and she just says 'i'm fine'. but i know she isn't. that's what she said last time. she has a good family and i think her school life is okay (she used to get bulllied but we sorted it out), so the only thing i could think of is that it could be herself.
i think i'm going to talk to her tonight about it.
does this sound like she depressed?
what can i do to help her?
thanks in advanced :)