I am neither a portrait photographer nor a user of flash, so I can't offer much technical advice in those areas. However, I would like to say that I quite like this image, in particular - the negative space around the girls (and the use of black especially). The space above the tallest girl's head is sufficient enough so her bent head appears to be an act of affection towards her sister rather than an attempt to cram herself into the frame, I even think the mild cutting off of the young one's behind works well. The affection between the two is also captured perfectly.
Not sure if this first attempt is by chance or thoroughly thought out effort, but for a first attempt, it is quite good in my opinion.