I'd say 'brothers-in-law' but I'm sure there are arguments to support both.
When people say sentences like "I seen that", it must really confuse non-native speakers. I had a friend from Puerto Rico who lived in England for a while, and he learned a lot from listening to our lazy, substandard-English conversations (although I'd never say "I seen that" myself). I'm not sure whether this was good or bad, but I suppose it broadened his knowledge of the language. The 'Queen's English', after all, is not the only way to speak the language.
There are a lot of occurances of this in English (as, I'm sure, in most other languages). It wouldn't, for example, be strange, where I live, to hear someone speak like this when telling a story:
'I went to the shop, and asked if they sold cheese. He says, "cheese? Yeah, we've got lot's of different types of cheese." And I'm like "well, what's your most popular type of cheese?" And he goes, "well, that'd be the white stilton with blueberries."'
This mixture of the past and present tenses is very common in colloquial English, especially when telling an anecdote. Furthermore, no one is 'like' an utterance, and we don't 'go', we 'say'. In spite of these inaccuracies, most people will be polite enough to speak properly when in the company of a non-native speaker. Unless they're a nationalist racist, of course, the chances of which are about one in ten.
-Nyarlathotep