The year is 1898, we’re in the late Victorian era, there are some very distinct characteristics to the fashion of the time, and we want the costumes to reflect that. No steampunk or modern anachronisms, please.
That being said: costumes are not what should stand in the way of participation. There is a lot to borrow in the community, and you can achieve a good look with simple touches. It is more important that the costume reflects the character than being 100% historically correct.
Laivgarderoben (The larp-wardrobe) has a lot of suitable outfits to lend (and is run by the organizers).
Inspirational boards and descriptions to come.