Ben Affleck and Jennifer Lopez have a lot of friends and family flying in this weekend to celebrate their marriage -- but one person won't be in attendance ... Casey Affleck.

Ben's younger brother, who's also a big-time actor, was spotted Saturday morning in L.A. on a Starbucks run ... where he looked pretty busy with his hands full. Of course, the obvious question that a pap posed to him was ... why aren't you in Georgia right now, man???

You can tell Casey wasn't really in the mood to chat much, but he does mumble something sorta telling as for why he isn't with BA today -- it sounds like he says he "fell asleep."

The cameraman tries clarifying to see if that's what CA actually said, but he doesn't respond further. In any case, it seems he's going to miss the festivities ... because just about everyone who's showing up is touching down in the Peach State as we speak.

Casey's been telegraphing this all week -- on Thursday, he was out with his girlfriend Caylee Cowan, and didn't appear that interested in talking about his sibling's big day this weekend.

Worth noting ... the Affleck brothers have seemed close over the years. But, they haven't been photographed together of late ... so who knows if they're all that tight these days.