My sister just became a teenager, so up until now she's been using kids sizes (140, 150, etc). 29" is her actual waistline, not her usual size. The only reason we chose the 22" because we more or less knew what fit her. Hell we thought she somehow massively fucked up her measurements when we asked her to over the phone, but looking back, 22" is tiny. Literally the circumference of my face.Vanity sizing is a thing, but it's typically standardized by brand, i.e.: your sister being a 29" in one brand doesn't necessarily mean she'll be a 29" in all of them. Also why women's sizes are typically single numbers as opposed to intuitive measurements found in men's clothes; apparently, it's easier for women to swallow saying they're a size "14" than saying they've got a 34" waist. It's why a lot of women are known for brand loyalty; they know "who" fits them.
But yes, I think a 22" jeans being loose on a 29" waist is a bit much, even for vanity's sake.