should i consider this as a hindrance in my practice
In my opinion, you have not lost your brahmacharya because your action was not intentional because your mind was in partial sleep. In Buddhism, for sin or transgression to occur, it seems there must be conscious intention. 

If you want to train in discipline like a Buddhist, you should never sleep with your sister, let alone with any other female, even your mother, let alone in public, because, for a brahmacharya, nocturnal emission can happen anytime. 

Buddhists do not believe in God. Sexual arousal is caused by chemicals, electrons &amp; hormones and only enlightened beings do not have sexual arousal. Sexual arousal is 'not-self' ('anatta'). 

The error you made was to sleep with your sister. As a brahmacharya, you must not do this because a brahmacharya should no longer behave like an ordinary householder.