Can a PhD student co-supervise master students in Germany?
By and large, yes.

The conditions for master thesis supervision are specified in the examination regulations of the considered department. Generally, there is a big variety between the regulations of different departments, so it's always worth having a look at them.

In all cases I'm aware of, PhD students can act as co-supervisors of master theses. Usually it's required that a professor is formally specified as the main advisor, both in the formal proposal and on the title page of the thesis document. The PhD student can be specified as an co-advisor on the title page.