As an update,
Trip went beautifully, had the time of my life with my amazing, gorgeous, brilliant girlfriend. After the first argument with Mum I didn't hear anything about S travelling with us but apparently behind the scenes Dad has stood his ground and point blank refused to change S's travel plans so she would be on the coach with us.
I've just had the most romantic, enjoyable, fun, loving weekend possible from the moment we left Mycity to the moment we returned. Not a single thing went wrong! I'm now hearing about Mum's denounciations of my "overspending" on this trip but I honestly would have paid triple what I spent to have such a good time!
Many thanks to eHell of course for helping my spine grow! A year ago I probably would have caved and babysat my older sister for the trip home.