Fred and George are Ron's older twin brothers. We don't see too much of them in the film, but they're basically the same: always joking, and always looking to keep things light.
When Mad-Eye gives them the Polyjuice Potion so they can all transform into Harry Potter's doubles, despite the gravity of everything that's happening, they can't help but crack wise:
MAD-EYE: For those of you who haven't taken Polyjuice Potion before, fair warning, it tastes like goblin piss.
FRED [OR IS IT GEORGE?]: Have a lot of experiences with that, do you, Mad-Eye? [He gets a withering look from Mad-Eye.] Just trying to diffuse the tension.
Mad-Eye isn't amused…but the rest of us are, so it's okay.
Despite their strong commitment to not taking anything too seriously, they do have some angst in this film: George loses his ear during Harry's transport. It's a heavy moment for the Weasleys, and Fred is certainly not laughing (for once) as he leans over his wounded brother.
Believe it or not, though, George is still laughing. Or at least, playing around. When Fred asks him how he feels, we're not sure what kind of answer we're going to get, but it turns out you can't keep George down for long:
FRED: How are you feeling, Georgie?
FRED: Come again?
GEORGE: Saint-like. I'm holey. I'm holey, Fred. Get it?
FRED: The whole wide world of ear-related humor, and you go for "I'm holey"? That's pathetic.
GEORGE: Reckon I'm still better looking than you.
We think he's going to be just fine. For now, at least.