According to your infrmation, you are currently still living abroad and would like to apply for family reunion with your daughter who lives with her mother and German stepsister/brother in Germany.
Theoretically, you can apply for a visa for family reunion with your daughter at the German embassy in your Country. This visa requires an exceptional hardship. which could be present, the family unity between you, your daughter, your German stepdaughter/son and their mother can only be conducted in Germany.
According to the Federal Administrative Court, the Possession of a resiidence Permit doesn´t prevent people (in this case your daughter) from leaving Germany for their common homecuontry and living together there.
Something else would only apply if Special circumstances which make a stay of your German stepdaughter/son in Germany appear to be the only alternative reasonable exist, in particular
- in the case of regular and personal contact between the German child and his German father, or
- if the German child would be harmed by leaving Germany, in particular e.g. because a chronic desease which can only be treated in Germany or due to a physical, mental od psychic illness or disability, Special educational or other reasons thechild´s needs make a further stay in Germany necessary.
In addition, the subsistence of your family, including yours, muss be secured. The visa granting is up to the discretion of the German embassy.