Dudley is one of my favourite secondary characters, because by the time we get to books 6 and 7 he's in such an interesting situation. In fact, his whole childhood is interesting. I'm really interested about the relationship between them when they're older. J.K. said that no child of Dudley's would ever have a smidge of magic... but I suppose he could've adopted. Or married a woman with kids from a previous marriage? There's hope
I don't know about in other countries, but in England, if you have family in the same school then when you get your yearly school photo you get one with your sibling, cousin or whatever too. I don't reckon Aunt Petunia would be too pleased though.