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If you're not talking about a VerpflichtungserklΓ€rung, then there is no set form or format to follow. Just write a letter in your own words.
Be aware that in this context, for a visit visa, there's really no such thing as sponsoring the visa application. At most you're sponsoring your relative's visit (that is, providing support so they don't have to fund it all themselves), and the purpose of your letter is to help explain to the consulate how they can afford to go. Nothing more.
In your letter,