Dear @Gago,
Welcome back to the Wefugees Community and thank you for your patience.
For a visitors visa, your mother will need to prove that she has a sufficient income. Otherwise, the person in Germany needs to "invite" and vouch for the visitor. Handbook Germany published an article on the so-called "Declaration of Commitment". It describes the conditions and requirements really well. Please follow the link for more details.
In any case, don't hesitate to get back to us with any further questions.
All the best,
Meike