Building a house on (partially) someone else's land

Please do not misunderstand. There are no future women. Both sons are single and not in any partnership.

So the ownership structures will then automatically divide like the land division? My parents are already landlords, so I would also enter as a landlord for tax purposes in the future.

So the ownership shares will then automatically split like the division of the property?

Yep, a separation of ownership and the property is not possible, as they are connected. Your parents and you would then have to pay taxes on the income from your brother's house. It is practically impossible to calculate that separately. You would also have to pay property taxes and similar jointly.

Therefore, my advice is to consult a professional (notary and lawyer).

That means all 3 parties would act as landlords? But wouldn’t it be like this: House1 belongs to Son1 and parents and is rented out = landlord House2 belongs to Son2 and parents and is self-occupied = not a landlord Thus, with 3 shares, Son1 would not be involved in House2?

But wouldn't it be like this: House1 belongs to Son1 and parents and is rented out = landlord House2 belongs to Son2 and parents and is self-occupied = not a landlord So, with 3 shares, Son1 would not be involved in House2?

NO, since all three parties are owners of the property. Therefore, BOTH houses are also owned by the three parties. You cannot simply separate the property and the houses. That is exactly the "nonsense" that this division causes.

No!
Read what wrote and understand.
Houses are merged with the property.
If ownership rights are not clarified, no financing bank will be found.

Please do not misunderstand. There are no future women.

And yet they are to be excluded already now, even before you get to know them. You wrote that this is the declared objective of the action.