Hi M.,
Sometimes, laughter is a way to release stress, so if it's happening at inappropriate times, this might be the cause (as opposed to thinking something is humorous). Has he received correction or attention for this action (inappropriate laughing)? If he has, he might be looking for more attention.
It seems to me that this has also morphed into being part of the bedtime routine for your stepson. If it were me, this is what I would do if I was putting the children to bed: start routine as usual, stories, snack and then, STAND UP (to be in a better physical position), explain that it's time to brush teeth, then leave the room with the cooperative kids and turn off the light. Just one reminder "Come brush your teeth when you are ready" and leave, no discussion, no coaxing, nothing.
What does this do? Well, it changes the dynamic completely, which is a start to changing what happens. Instead of the grabbing games (standing away from the child will help), etc., daddy is in a different room with the other kids. NO ATTENTION. As the adults, you may have to call his bluff.... which means letting him sit in there, laughing and hollering as he pleases, which will likely happen if this is based in attention-getting. If he comes out and is uncooperative, sit him on a chair out of sightlines (but not hearing) and just tell him "You must sit here until you are ready to cooperate and brush your teeth." Then, keep returning him to the chair if need be, but try to ignore as much as possible.
He may fall asleep on the couch or chair. If the chair is big enough, don't move him. Just cover him with a blanket.
My son is less-than-cooperative at times,esp. bedtime. Our routine is such that pajamas, toileting/toothbrushing, etc. happen before stories. If we have great cooperation, we "have time for three stories". Any more than two petitions on our end for his cooperation, then we "only have time for two stories now, because you are taking too long to get your jobs done", etc. We've only had a couple One Story nights since we started this very natural consequence about a month ago, and we've found this has been pretty motivating for our little guy. (He's 3.5)
I'd also make sure your late snack isn't a 'treat', as sugar can sometimes play into things, making a tired child seem overactive when they are, in fact, often overtired.