cdm2003 wrote: My Belgian relatives insist that Flemish is not Dutch
When my Belgian cousins start speaking West-Flemish, there is no way I (speaking Dutch) can understand them. I wouldn't call it Dutch myself. The official language in Flanders (a part of Belgium) is Dutch, but dialects are spoken. Some see Flemish not as a dialect, but as a language on its own.
You can't understand them because the vocabulary is different or because the pronunciation is different even though the vocabulary is largely the same?
If someone from Brabant doesn't speak clearly I have trouble understanding him although pretty well the same words are used.
The Netherlands and Belgium have so many dialects (not only pronunciation but also words that occur in one but not in another dialect.) that people jokingly say the tower of Babel was located there.