Refusing costs between -25 and -100 prestige. It will also cost you a -25 on how they see you, so it makes it unlikely they will come and help.
Accepting means you are at war as long as they are. Being at war cuts out some of your options, like giving titles to those outside your kingdom, doing most of the feasts or tourneys and so on.
That said, they usually tell you that they can’t come and help most of the time. Also marrying off daughters to the big factions usually works to your disadvantage. They are always at war, this makes them less likely to help you and much more likely to call you. On top of that your daughter is likely to die just as soon as a male heir is born. Offspring are not your allies either. They also have a weak claim on your lands or kingdom.
Not that that will matter so much. Sweden is going to get stomped by one of the hoards when they come on the map anyway. It is just about as bad as trying to play in Spain.
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