Police say the sisters, ages 16 and 22, were attacked on West Drive at West 106th Street at 9:45 p.m.
Two young men, believed to be in their teens and wearing ski masks, held them up with a black handgun.
The suspects took their cell phones and their wallets and fled.
So far there have been no arrests.
The tourists, from Camarillo, Calif., were not injured.