They may not need to be in quarantine - but it seems they would not be allowed to enter the country at the moment in order to visit family.
https://www.bundespolizei.de/Web/DE/...00317_faq.html
Grundsätzlich liegen in folgenden
Fälle keine zwingenden Gründe vor:
Besuch von Verwandten, Bekannten oder Freunden
All of the governments involved are still advising against non-essential travel.