What a surprise!!

At first I was a little confused, but now it makes sense. It's all in flashbacks, except for the first panel with the three mini panels in the top right corner.

My theory is that Bau's "twin" was angry with his brother when Karl and Bau first met, and that Bau made music with Karl instead of his brother and only spent time with Karl. Anyway, we now also know that Bau's brother's name also starts with something B. ^^

my interpretation is that the brothers were both into music initially, Bau on guitar and brother on harmonica, to the point Bau wanted to make a Musical duo with his brother, but when their dad showed them the farm equipment and stuff, it looks like the brother got more interested in that than playing with Bau, and eventually Bau met Karl, who shared his interest in music.
This is where it gets a little confusing since we've only been shown certain things so far. The brother shouldn't be mad that Bau is playing with someone else, because the brother was the one who stopped playing with Bau in the first place, according to these panels.
I think the brother isn't angry that Bau went off to play music with someone else, I think the brother is mad that Bau didn't join in the "family business" taking care of the farm or he felt abandoned because Bau left them to pursue his interests in music.
it's all theories tho, based on the (somewhat messy) imagery we've been given so far.

Makes a lot of sense!

Alguien mas siente que el harmano entrara a su cuarto viendo que alguien esta en el lugar y momento equivocado?

there comes a time in every boy's life where he either discovers the allure of small electronics or large mechanics and there's nothing you can do. you are pulled to industrial farm equipment against your will.

I'm hearing "Goodbye Yellow Brick Road" while reading this.