Is it permissible for a Muslim to inherit from a Christian German family that adopted him? And is it permissible to accept money from them as a gift, and should he use it or give it away in charity?

It is not permissible for a person to ascribe himself to anyone other than his father, due to the Almighty's saying: "Call them by [the names of] their fathers; it is more just in the sight of Allah." And due to the Prophet's (peace be upon him) saying: "Whoever claims to be the son of someone other than his father, while knowing that he is not his father, Paradise is forbidden for him." Therefore, whoever has done this must repent to Allah Almighty. As for your inheritance of that woman's money, it is not valid due to the absence of a relationship between you. However, if she gifted it all to you, there is no harm in accepting it and benefiting from it, just as her bequest to you is valid. If this happens to you, you should give charity from it and spend it on good deeds and avenues of righteousness.