They're certainly related, but I find Opium to be much spicier, and with a strong carnation note. Youth Dew is softer and rounder, maybe not as opulent as Opium but more sultry and sensuous, imo. Youth Dew is a favorite of my fiancé but he's ambivalent about Opium - he likes it but he thinks it smells a bit too masculine(!) I don't think it's masculine, but it does have that clean and soapy barbershop feel to it, which Youth Dew doesn't have.
I think you might actually be talking about EL's Cinnabar. Cinnabar came out in 1978, a year after Opium, while Youth Dew came out 25 years earlier, in 1953. I would say that Cinnabar and Opium are sisters, maybe even twins, and Youth Dew is their cousin.