Buy parents' apartment below market value

There are actually people who are not consumed by envy. Maybe she doesn't even live in Munich, doesn't rent, has a job where she earns a lot, has already received money.... There are so many possibilities.

Between blindly loyal/naive and envy, there is something in between. It is called justice and fairness. And even if nothing has been repaid yet, the brother now has the chance to buy an apartment at half the usual local price. Of course, that is a large gift, which is unfair and unjust to the sister if she does not receive the same amount.

And the sister is being ignored here.

But that is not firmly decided at all. The OP is considering how to make it fair. There are various possibilities that could be considered. However, I would also recommend thinking about it right away, so that it doesn't just hang in the air. So, somehow something like whether the other person is paid out (which doesn’t necessarily have to be done in one payment) or if you somehow set up something notarized that then concerns a possible future inheritance (there is also a house, so certainly other assets besides the apartment in question).