Really good for a first attempt, I like it that she looks friendly yet thoughtful, happy but with a touch of melancholy. A portrait should always show us more then "just" the face of a human, it also should reflect the character a bit.

Of course, that's just my interpretation, so if I'm wrong I'm terribly sorry, mate. Anway, you should consider to make a self-portrait - or maybe a siblings-portrait?
I'm sure you and your sister will be happy about it, especially when the years go by and you both can always look back on it.