The modern spelling of Wærdalen is Verdal, a municipality in Nord Trøndelag county. Lillesan sounds like Lillesand and there may be such a place in Verdal. I have found about 21 people in Verdal on the 1900 census with the name Ydse but since you gave no names, I didn't pursue it. That census is the latest one available.
Go to: http://draug.rhd.isv.uit.no/folketellinger_engelsk_britisk.h...
Type Ydse in the surname slot, scroll down and check the Nord Trøndelag box and hit 1900. You may find them. Let me know if you have any problems.