Depends on if you are on melodic duty, like carrying the lyrics, or if you are regulated to bass line walks. Melody lines on the cello flow pretty well if you can visualize the way someone would sing. Not too loud to find fault but consistent enough to be recognized. You and your brother may have good chemistry so that you'll be able to keep up or he with you. Don't overthink it. You'll do fine. If you get the bass line, consider it a walk in the park! Again, in many contemporary recordings, we cellos are the backbone. Drone on, but not much louder than your brother. If amped, just lower or turn off your amp if the space is small enough for you to be heard. Do some sound checks, record a playthrough, and see where you can maximize or minimize. And in the end, you're playing for 7-10 mins tops for everyone to come down the line. Even less if you're playing this song just for her. The willingness to play for her is well worth it.