Is this Legend of the Galactic Heroes (FUJISAKI Ryuu)...?

Five hundred years ago, Rudolf von Goldenbaum united the disparate governments of humankind and established a mighty human empire, a tribute to his own greatness that stretched across the galaxy. Demanding loyalty and discipline, Rudolf created an order designed to long outlast his own rule; but all human works fade in time. Today, the decaying empire faces the threat of the Free Planets Alliance and military leaders on both sides jockey for power in the face of a seemingly inevitable conflict. On the side of the empire, the young general Reinhard von Lohengramm sees war as an opportunity, and the empire as soft clay ready for his own sculpting. On the side of the alliance, would-be historian Yang Wen-li would like nothing better than to live out his days in peace - but times of war have little use for men who waste their days studying the mistakes of the past. Circumstances will make heroes and villains of both these men, as they write new history one blood-splattered page at a time.

In the first chapter, we're introduced to two boys; Siegfried Kircheis, a dark-haired commoner, and Reinhard von Musel, a fair-haired boy of noble descent who's described as being 'beautiful' in a caption.

Reinhard has an older sister named Annerose, and Siegfried quickly becomes friends with both of them, but Annerose is subsequently sold by her father to the Kaiser, under threat of being executed if he refused. Just before being driven away, she kisses Siegfried on the forehead and tells him that she baked a chocolate cake that morning.

Reinhard later enrols at the Imperial Military Preparatory School and convinces Siegfried to join him, so that they can take over the universe and get his sister back.

Much later in the story, there's a space battle involving a fortress with a powerful gun referred to as Thor's Hammer.

Is this Legend of the Galactic Heroes (FUJISAKI Ryuu)...?

Five hundred years ago, Rudolf von Goldenbaum united the disparate governments of humankind and established a mighty human empire, a tribute to his own greatness that stretched across the galaxy. Demanding loyalty and discipline, Rudolf created an order designed to long outlast his own rule; but all human works fade in time. Today, the decaying empire faces the threat of the Free Planets Alliance and military leaders on both sides jockey for power in the face of a seemingly inevitable conflict. On the side of the empire, the young general Reinhard von Lohengramm sees war as an opportunity, and the empire as soft clay ready for his own sculpting. On the side of the alliance, would-be historian Yang Wen-li would like nothing better than to live out his days in peace - but times of war have little use for men who waste their days studying the mistakes of the past. Circumstances will make heroes and villains of both these men, as they write new history one blood-splattered page at a time.

In the first chapter, we're introduced to two boys; Siegfried Kircheis, a dark-haired commoner, and Reinhard von Musel, a fair-haired boy of noble descent who's described as being 'beautiful' in a caption.

Reinhard has an older sister named Annerose, and Siegfried quickly becomes friends with both of them, but Annerose is subsequently sold by her father to the Kaiser, under threat of being executed if he refused. Just before being driven away, she kisses Siegfried on the forehead and tells him that she baked a chocolate cake that morning.

Reinhard later enrols at the Imperial Military Preparatory School and convinces Siegfried to join him, so that they can take over the universe and get his sister back.

Much later in the story, there's a space battle involving a fortress with a powerful gun referred to as Thor Hammer.