I think I can help you, because I did this quest an hour ago
You need to fight and capture a swedish lord, and he will talk to you about the kidnapped bride as soon as you keep him as a prisoner. Then you could just go to the fotress where she is held custody and pay a certain amount of money to get her out.
Btw, maybe you could help me on the other quest "Horse Thief"? I'm stuck in this one, it's also one of the quest given by the three polish potential supporters.