JD Wetherspoon is a chain of hotels and pubs, so he means that the brother is a little too vulgar for his age or for the occasion.

Well, correct me if I'm wrong, but Wetherspoons is a very informal place where you usually get drunk and don´t really need "to care" about what you're saying. You don´t need to behave formally. While at a dinner party, it is the other way around. It is a formal situation where there are usually relatives or in-laws. So you need to watch your language a little. Even if there are no relatives or in-laws, the dinner party is usually thrown for a specific reason. Sometimes even bosses are invited to dinner parties depending on what they are "celebrating". There are occasions for everything.

Weatherspoons is a budget pub chain in England. You pay (very) low prices for beer. This often means their pubs are not that classy. So, the type of humour you would hear in a Weatherspoons is not the same as you would hear at a dinner party. You might get a few rougher or more sexist jokes going on. Russel Howard: neither dinner party nor Weatherspoons funny in my opinion ;)