Why is my step-daughter having a blood test for von willebrand disease?

Bleeding history? Doctors would typically order a test for VWD if the patient had a history of abnormal bruising and/or bleeding that made them suspicious, especially if there was a similar history among other members of the family. You don't mention how old your step-daughter is; the question is probably best directed to your daughter if she is an adult, or to her doctor if she is a child.
Bleeding. A physician thinks for some reason that your step-daughter has shown an unusual tendency to bleed, or one of her relatives is known to have von-willebrand's. The harder you look, the more vw's you find; some say 1 person in 100 has a mild version. It's good to know, usually not a big deal.
Easy bleeding. Von willebrands is the most common bleeding disorder. The reason for testing for this is some sort of abnormal bleeding. This includes frequent nosebleeds, gum bleeding, unusual bruising, or heavy periods.