Ha ha, people on here are sooo mean!
Just speak too your sister NOT in front of your niece and say, have you noticed X smells? Then say, your sister needs to speak to her about body hygiene and deodrant etc, so that she doesn't get bullied.
I realised recently that I don't need to use deodrant because I don't smell when I sweat! Apparently, a small number of Europeans have a gene that means they don't produce a certain protein in their sweat that attracts the bacteria that produces BO. Sounds disgusting but the way to tell is if you have only dry earwax, which I do! If you do too, you don't need deodrant.
<a class="postlink" href="http://www.guardian.co.uk/science/sifting-the-evidence/2013/feb/14/wasting-money-deodorant-ears" onclick="window.open(this.href);return false;">http://www.guardian.co.uk/science/sifti ... orant-ears</a>