Inheritance semi-detached house - Will or land register decisive?

Logically they are visible. I suspect they were entered upon the grandfather's death and then no one ever took care of it. Now the grandmother is dead and according to the will, only the two sons inherit. This will then be executed with the portion of the grandmother that still belonged to her. But the 3rd son registered in the land register remains with his share as originally received.

Yeah - indeed. Inherited from the late grandfather back then: grandmother 50% and the other 50% divided equally among the three sons. The grandmother's 50% now, except for the compulsory portion of the third son, goes to the two sons according to the will. So it remains a community of heirs - that's a pain. According to my calculations, the two sons should jointly own 75% and the other son 25%. Then it will become a very difficult matter. As the grandson, I am supposed to buy the house at a reasonable price - unfortunately, this will only be possible with the consent of the third son, and no one knows how he thinks nowadays. If he blocks the matter, it will probably be a lengthy process.

So it remains a community of heirs - that's crap. According to my calculation, the two sons should jointly own 75% and the other son 25%.

In my opinion, it should be 33.33% x 50% = 16.66% and for the other two 33.33% * 50% + 50% * 50% = 41.67%. But that doesn't make it any better.

For the third son, it should be:

Upon the death of the father, 50% to the mother and of the other 50% 1/3 = 16.67%

Upon the death of the mother, the compulsory portion of the remaining 50% = 1/3 of 50 = 16.67%, of which, because it is only the compulsory portion, only half, i.e. 8.33%.

In total then 16.67% + 8.33% = 25%.

Where is my mistake? :)

Have you checked that?
It could also look like this:
50% grandma, 50% grandpa before the first death. After that, 75% grandma, 25% community of heirs.

Did you check that?
It could also look like this:
50% grandmother, 50% grandfather before the first death. After that 75% grandmother, 25% community of heirs.

Since the death was very sudden and at a relatively young age back then, I am 99% sure that nothing was regulated regarding the inheritance.
Then the law would apply – according to my research, it was 50% spouse, 50% children at the first death.