You have to tread carefully with this kind of search.
Assuming the half brother is still alive, he may not be aware that the man who brought him up was not his biological father - you could be stirring up a hornets’s net. Similarly for any of his relatives or descendants if you find DNA matches to them.
On the other hand, if you find a DNA match who does not want any contact with your father in law, this could be very disappointing for him.
Just a few things to consider.

If you are looking exclusively for matches who live in Germany, or have German surnames, you might be limiting yourself - people move around and women marry and take their husband’s surname, so maybe expand the search? Many of my matches have come through the female line.
Good luck.
