Help with coop on game already in progress

My brother lives overseas and is starting BG3, whereas I have a little over 800 hours logged (0 multiplayer). Our play times don't always align, but rather than just spectate him, I'd like to be able to join his game on occasion and inhabit one of the origin characters. We tried this the other day but couldn't get it to work my question is:

Is it possible to join a friends campaign that is already in progress and just pilot one of his party's origin characters?