Can I ask how you know those things? Knowing the source of the information will be a great help when it comes to pointing you in the right direction for more information.
I would like to find his brother's marker
Can I ask what information you have that there is a marker? Back then it wasn't usual for German soldiers to get their names on their graves. The Soviets didn't even bother to give their own troops the kind burials that the US and Commonwealth armies did; disposing of dead Germans was almost exclusively a public health issue. Your idea is a very nice one, but if you're expecting to come to Poland and find anything like the very moving Colleville military cemetery, you're very much mistaken.
Any help on where to start would be appreciated.
It seems that for graves info you need to contact the People's Union for Care of War Graves. Their website is at volksbund.de/en/volksbund.html but apart from that one page it is all in German. There is an online search engine on the site but it makes no mention of any cemetery within 120km of Warsaw. However, there is a form for making an online request for any information they have about any grave or memorial.
From a quick look at my usual sources, I can only find any mention of two German WWII war cemeteries: one actually in Warsaw and one in Modlin, which is about 30 miles north of Warsaw.
Details for the first are here: en.tracesofwar.com/article/3476/German-War-Graves-Warsaw.htm
Details for the second are here: en.tracesofwar.com/article/3471/German-War-Graves-Modlin.htm
If you haven't also already contacted them, you might want to email the Deutsche Dienststelle, they have records about persons who served in the German armed forces. Their website is here: dd-wast.de/en/home.html