Do you guys talk on steam with FO:NV with your friends?

I have a brother with an Xbox 360, and I have a PC with no friends. I'm basically looking for a way we can interact over a distance while gaming, if possible, in -game.

I understand that the games are made for specific platforms, and the gamers are kept separate depending on platform for technical and capital reasons.

Also, is it legal, if he comes to the house and plays Fallout:NV on my computer in my account? And my Steam library of games? And I can play on the Xbox as his user for a time? Or do we need to set up a steam account for him on my PC, and a gamertag for me on his Xbox each time?

One more thing, I noticed on Xbox that when you get an achievement, a graphical overlay appears to show as such. Since I've only played Fallout: NV, I've noticed these when it dings a quest complete, or challenge, or something I pretty much feel is an achievement it's shown in the top left via console transaction. But I got a new game (Trine), and it doesn't show achievements. How do I enable the achievements to popup in steam when I'm gaming or does this not exist? Like I did the one where you use a wizard and make a tower with PhysX on PC with an ATI card... gently lol... and stand on it, but I didn't know I got it until I win the level and quit the game, look into steam, and click the +15 (or w/e) to see the achievements you got and see the one before the last one. ?Basically I can't figure out how to have steam games show achievements, perhaps it's because I tweak the setup graphics options before playing.