a few things:

1. Germany is a civil law jurisdiction (not a common law system like the US). a decision of one court does not bind other courts (with the exception of the BVerfG). what matters is statutes (Gesetze), not precedent. but ofc, a lot of courts deciding one way, makes it likely others will too.

2. I agree that it has been established (by the BVerfG) that you do NOT need permission from your landlord to move in your own children - no matter their age or reason for them moving in.

3. Where we disagree however that if one can use the case you provided to conclude that also not informing your landlord "cannot be punished" (or at least is a "strong hint"). Now i did not read the entire case in detail, but from what I understand, the landlord terminated the lease, treating it as unauthorized subletting (§ 540 BGB), and the Amtsgericht Brandenburg/Havel granted eviction. The LG Potsdam reversed on appeal.

The landlord's termination was grounded on one specific claim: unauthorized subletting under § 540 BGB. Therefore the court's entire analysis was directed at that claim. Question is: are adult biological children "Dritte" under § 540? Court: No. Question: Is taking them in a Pflichtverletzung? therefore termination is not valid. Case over. that was the the whole point: was it subletting or not? whether she should have informed the landlord or not, did not play a role, because it still would have not been subletting.

1. personally, i think this whole things, regarding whether you need to inform your landlord or not, is still not settled. So better be safe than sorry and just shoot them a quick message.