It's not - but much like Electoral Saxony Brandenburg had a relatively laissez faire attitude to the war. IIRC there was a struggle going on between the pro-Habsburg chancellor (heading up the pro-peace party) and the electress, sister of Bedrich, king of Bohemia and her son, who were the pro war party. So, perhaps with his daughter/sister directly involved because he has a more vested interest in the outcome of the war (sister married to the king of England, married to the sister of the Bohemian king, and sister of the king of England married to the king of Bohemia), Brandenburg actually gets off its ass and does something (IDK what exactly) but not the OTL fence-sitting that they did. Same goes for Saxony. A match between one of the elector's sisters/daughters and the future king of England can bring them down off their perch to throw their lot one way or another.
As for Gustav Adolf, well, I'm sure there's another Brandenburger sister he can marry, or maybe, he gets elected as Tsar of Russia. Or, maybe one of Sigismund III's daughters can survive and he can marry her. OTL he seemed very interested in marrying the Winter Queen, Elizabeth Stuart (her mother not so much), and following her, he settled on a match with Maria Eleonore. But his mother also forced his hand somewhat, since she didn't want him to marry a commoner (and she played a big role in having his brother's marriage declared morganatic when it was discovered (despite the fact that AFAIK such a thing (morganatic marriages) didn't exist in Sweden at the time).