Just for viewing in the land register:
what you are looking for is stated in §12 GBO (Land Register Ordinance), if you can prove a legitimate interest, you can take a look at the land register sheet, all viewings are recorded.
I would classify your described case as a legitimate interest, the responsible judicial officer ultimately decides independently.
It is true, you are allowed to view the land register if you present a legitimate interest, § 12 GBO (as already mentioned). However, in my opinion, the OP has NO legitimate interest. He is neither the owner nor does he have a secured right on the property (section II, section III, for example usufruct, land charge), nor are there any other reasons that constitute a legitimate interest. The fact that the house belongs to his parents or his brother is completely irrelevant and does not justify a legitimate interest. Nor does the fact that he might possibly inherit this house at some point in the future or not. In my opinion, this is definitely not a legitimate interest within the meaning of § 12 GBO.
I am a judicial official by profession. If I were assigned to the land register department (which I am not), I would have to decide on the legitimate interest for viewing the land register. In my training (which was not long ago), it was made very clear to us several times that neither a child nor a sibling has a legitimate interest in viewing their parents’ land register during the parents' lifetime. It is completely irrelevant whether the OP might inherit the house someday or not; this is not foreseeable at the current point in time. The situation looks somewhat different after the inheritance event has occurred, even if one has been disinherited and only entitled to the compulsory portion, because then the legitimate interest is justified, for example, by the need to calculate the compulsory portion based on the property value and to know of any encumbrances entered in the land register. But during the parents' lifetime, there is no legitimate interest! Not even a spouse has a legitimate interest in viewing their spouse’s land register (if the property is owned solely by one spouse)!
Basically, every responsible land register officer must decide for themselves; I would not grant access in this case. However, I do not know how the officer responsible for the OP will see it. Maybe he is lucky and gets access... maybe not.
That means the viewing is recorded but the parents are not informed?
If you get access: Yes, exactly. The viewing is recorded, but no notification is sent to the owner.