Go in her closet &find out what SIZE SHE WEARS. For example: a size medium dress could fit a 17 years old girl, or a 61 years old woman, and not all SIZE MEDIUMS fits all. Sometimes, they look like they're size double zeros. I'm a size medium, and sometimes a size 10 fits me perfectly, depends on the style or brand.
I haven't looked at the link, but I'm guessing it's a picture of your sister... Do you think it's right to post her photo without permission? There are a lot of weirdos out there - did you think that perhaps she doesn't want her photo all over the internet? I would advise you to remove it straight away. Also, it's impossible to guess someone's size through a photograph. You would do better to ask her in a crafty way or look through the sizes in her wardrobe.
a good one about maybe size 10 uk
