My sister was born in 1997 as a Bulgarian citizen in Germany. She became a German citizen while she was rather young (5 years old if I remember correctly), while keeping her Bulgarian citizenship. Both my parents are Bulgarians but near the end of 2002, a few months after my sister, me and my parents also became German citizens but had to give up our Bulgarian citizenship, from what I understand between me being born(1994) 1997 there was some law change that allowed my sister to keep both citizenships
She recently tried to get her German passport renewed but was told that they could not just give her a new passport because she was what is called an "Optionsdeutscher" and in a year would have to choose whether to keep her German or Bulgarian citizenship.
Now from what I understand and according to this government website
she has the right to keep both Citizenship's because while we have lived outside of Germany for a long time she fulfills the 8 years of living in Germany and also the only other citizenship she holds is the citizenship of an EU member country.
My question is whether my understanding of the is right, and if so what legal steps can be taken in order to be able to keep both citizenships?