your niece or nephew will keep its German citizenship after her 21st birthday, if she is raised in Germany,
Problems with the citizenship arise only if a child goes to another country for a longer period, e.g. if he or she decides to live with the grandparents or to attend high in Rwanda.
The case is not completely clear to me, so if you like, you can submit the letter via MBEON, which is a safe channel to convey confident information.
The registry office doesn't interfere in citizenship matters, so I suppose the letter was sent from the immigration office?
Please feel free to contact me again. My name is Christine, I#m
working in Munich and I would be happy to answer your further questions. In case you need a confidential consultation you can also write me at mbeon, which provides advice for adult immigrants in Germany via an app. Download the app at Google Play or App Store and get in touch with me or 160 other migration advisors. It's free of charge, anonymous, data secure and you can get advice in 15 different languages. Find more information here: ***.mbeon.de.