Actually her parents, or at least people claiming to be such do appear in canon, if barely. They get no dialogue, but during the shopping trip in the second book there's some mention of them being uncertain around all the magic and weirdness, Arthur Weasley saying something along the lines "oh wow, I get to meet real muggles, look they're exchanging muggle money!", and few lines about them being unnerved by the confrontation between Arthur and Lucius in the bookshop.