You can word it pretty much how you want to your sister, providing the daughter doesn't hear. Say something like 'alright our kid, not being funny but your daughter stinks. Only saying so that she doesn't get targeted at school'.
To the point.
Bloke in our office used to stink. He was a good sport, so we all chipped in to buy 20 magic tree air freshener's and just scattered them all over his desk. He took it well, but still stank afterwards. He must have just not given a single fuck.