So my brother is kinda interested In Fallout 4 (I think) the thing is he has never played A fallout game before Now of corse I would love him to play and enjoy this series as much as we all have . I have a boxed copy of FO3 GOTY and I was thinking would it be better just to re buy it on steam or should I just reinstall it ? I think Jon will enjoy it if he get's into it or is he better of just starting on NV (in my eyes the best game of the series)? Thanks for your advice

If you already have the game, don't pay twice for it. You can add your existing copy in your Steam library manually (iirc; though I don't remember how it is done) if you want it there. And just make your own recommendation to your brother. If you like New Vegas better, recommend that to him. Fallout 3 might feel a bit more simple than it already is after New Vegas, but it shouldn't matter.

Fallout 3 first then Fallout: New Vegas, playing Fallout 3 second would seem like an anticlimix imo with the inferior story, characters and writing.

You should play fallout 1 and 2 if you haven't they are also fantastic games