My sister said that it is satanic because you kill someone inside the game

Yes, but they're in it for the money. No real religious reason

The dark brotherhood do it for money. No real religious reason (there is however a creed and a prophet)

The Dark Brotherhood are simply assassins for hire. Evil is a matter of perspective.

I say we handled that really well

Short answer: no

Survival- not really. It's more of a kill anything you want game 

As toxi said, evil is perspective. There isn't a hell

Wow- we massacred that idea

The only thing that is even close to being "satanic" about it is encounters with daedra. Apart from that, it has no relation to christianity. Demons and daedra are actually seperate from each other.

You kill LOTS of people!!!

Yeah. It's her opinion. But for me, it's just a survival game where you must live.

 satanic? Well the daedra are the only things close but they are a seperate idea from a "Demon" as well as skyrim haveing no relation to the judeo-christian religions.

So, no.

You can capture souls, and use them for enchanting rituals, so if you read into it enough...

http://www.uesp.net/wiki/Skyrim:Aedra_and_Daedra

There are some instances that can be considered satanic (see the wind helm killer or daedra rituals) but killing is not one of them. Most games include a killing engine, this means that most games are satanic? And also, if anything you stop daedra style beings. 

The game pretty much comes out and tells you, on many occasions, Daedra are evil.

And even then not all daedra are pure evil. If anything the dragonborn is an anti satan person

Is the Dark Brotherhood not evil?