@Binary175,
At a very young age, we are attached to our siblings. This can become overly intense, especially when the parents are passive, absent, alcoholics, distracted, unhappy, mentally ill, ignore the needs of the children, etc.
Then we see the outside world and want to explore that, including other people. If we had good relationships in the home and parents did a good job, then we have the courage to greet the world and fill our needs outside of the family unit.
You have not grown up from the time you and your sister - as little children - used to cuddle and watch cartoons. Now, it's on to porn and you are entering a dangerous terrritory by creating a sexually stimulating environment when you are with her.
You should be watching porn with a willing girlfriend, not with your sister. No good can come out of acting out your childhood desire to get close with your own sister in a sexual way.
Your path to maturity got stunted along the way. Grow up and be a brother to her.