it's the free will of the stepfather to decide whether his stepdaughter will receive anything.
Adoption has the disadavantage that it takes some time, if you cannot prove the close relation between the 2. However, this should be possible if both have been living in the same house for a considerable time in a "familiy".
There is the risk of death before concluding adoption. Advantage is that in Germany 400000 Euros are free of inheritance tax (instead of only 20000). Thus, the stepfather would be throwing money out of the windows in giving the daughter a lot of money or other assets without adoption.
Of course it would be possible to modify the last will (testament) without adoption so that the daughter receives a greater part of the assets. This is no problem as long as the miminum inhertance (Pflichtteil) of the other parties who would inherit without any last will are respected.
The stepfather should call a notary because in any case (also adoption) it will be necessary.
If the stepfather has a lot of assets and no considerable depths, as stepdaughter I wouldn't hesitate too long to accept adoption although her family name would change (which in case of a married person using her husband's surname wouldn't be a too great sacrifice).