Hi @Mashoko
Yes, your relatives can apply for a short-term Schengen visa for the purpose of visiting you. They will have to book an appointment at the German embassy where they live and provide various documents to support their application. Here, for example, is the checklist of required documents for people wishing to apply for this in Nigeria:
https://nigeria.diplo.de/ng-en/-/2566564
Proof of financial security is one of the things they need to show. Either they can show that they have enough money to cover their costs themselves, or you make a special declaration that commits you to covering these costs. This is called a ‘Verpflichtungserklärung’ and is done at your local Ausländerbehörde.
Best,
Éanna