Billie Eilish's Brother, Finneas, Reacts to His First Headliner Show

Billie Eilish's big brother says he looks up to his little sis for inspiration now that he's taking his own music career on the road ... even though he's already the brains behind her brand.

Finneas O'Connell talked to us Wednesday at LAX, just hours after his first big solo show -- sans Billie -- out in SF ... where it looks like he killed it all by himself.

The guy's a first-rate musician, and if you didn't know ... he actually cowrote and produced most (if not all) of Billie's tracks on her album, "When We Fall Asleep, Where Do We Go?"

Considering he has usually accompanied Billie onstage when he's performed until now, we were curious how he handled the audience, and if he took any cues from Billie herself ... seeing how she's got incredible stage presence and knows how to rock a crowd.

Turns out ... Finneas actually took diligent notes on how Billie handles herself during a performance, and it sounds like he applied some of those techniques for his own gig.

Now, for the juicy question ... does he, ya know, resent folks who might show up to his shows just because they're Billie fans? Check it out ... Finneas is a class act, telling us he'll take the support either way, but he DOES appreciate one type of fan in particular.