My best friend is being abused...
My friend, who I will call Sienna, is being verbally and physically abused by her parents. She often tells me about how they make fun of her artwork, looks, and of her watching Star Trek. She has shown up with bruises on her face and body. Her parents will ground her for stupid things like having a bottle of water in her room or for getting a C grade in a class. Now, I have never met Sienna's parents, however there is plenty of evidence to back it up; her fraternal twin talks to me about Sienna. The two of them hate each other. Her twin, who I will refer to as Laura, is evil: she has stated several times to me that she enjoys war, killing, torture, etc. She has even told me that she has decapitated birds, which is something I couldn't make up. Laura is the favourite in the family of 6 (4 kids), and is possibly a sociopath. So, she must be telling lies to their parents about Sienna to get her in trouble, since Sienna is very easy to get along with, and would never hurt anyone.
After we started freshman year, I got her number, and she got mine, so we spoke quite often. Now, note: I am a straight-A student with a perfect record. I consider myself to be a kind, easygoing, and polite person who can get along with almost anyone. I've never had alcohol, drugs, or s** with anyone ever in my life. Anyways, after about a month or two of talking to each other, I heard from Sienna that her parents thought I was a "bad influence." Naturally, I asked why... She said that she didn't know why, that maybe they thought she was skipping school with me. But that wouldn't be possible, since my parents always know where I am and who I'm with. Heck, they barely let me walk my own dog around the block by myself (Overprotective parents, gotta love 'em). Anyways, after this I learned that they don't let her paint (she loves painting), watch television (her only source of Star Trek is on H&I), or have friends that are "bad influences" (AKA: Kind, wholesome people that support her). Now, all that I've mentioned is terrible, but the thing that REALLY makes me mad is the fact that Sienna is the only one that is treated like this out of four kids in the family. Now, I'm happy that they're not being abused, but why single out Sienna?!
How should I handle this?