Sibling attraction is actually more common than people realise, with varying degrees. Basically it's a female of close age bracket that you'll likely have some common interests with. On top of that it's someone you speak to regularly. It can be anything from overly fond (BFF) to lust to full blown love feelings. So don't think yourself unusual outright.
But, you do need to change your mindset.
What you need to do is called cognitive behaviour therapy. Right now, you've associated your sister with a sexual fantasy. Every time you think of her that way, you're going to reinforce that idea and you'll make it worse. Over time it'll cause you embarrassment when you speak to her and worse still should you try to talk to her about it (she will NOT understand).
You need to do two things for CBT. Firstly, you need to logically realize the problems this will cause. You need to actively think about all the problems of having your sister as a sexual fantasy. Once you realize the problems this makes, you need to remind yourself of them again and again. Secondly, you must stop fantasising about her / masturbating to her. Should it come up you need to think about the problems it will cause. Find another fantasy, another girl or celebrity etc that works as well. Thought processes are like muscles, and only by changing your mindset time and time again will it get easier. Also, you have to delete your copies of the videos because you won't have the will power otherwise.
It may be a good idea to see a counselor or psychologist. They are trained in CBT (amongst other things) and can help you work through it and the issues you have with it.
Best of luck.