Mr. Nice was started by 1 of the original founders of Green House Seeds and the strains are similar but with better genes. Great seeds.
Shantibaba was the original Founder/Owner/Breeder of The Green House Seed Company. He took on two partners, one was the present Green House Seeds owner Arjan, who originally handled the sales end of the business.
Sometime after Shatibabas White Widow won the Cannabis Cup tensions began and egos bumped into each other and the trio split up with Shantibaba and the other partner selling out to Arjan.
Even though a company each breeders genetics remained their own property. What they had brought to the business and what they created from it remained their property and not the property of Green House Seeds so when they left their genetics went with them leaving Arjan without the original genetics to use to attempt to duplicate the strains lost to the line. Like other breeders lacking original genetics but wanting to cash in on the strain-name he created his own versions and gave them the same names.
Shantibaba opened Mr. Nice Seeds, in conjunction with Howard Marks aka. Mr. Nice, and another breeder working with him. To try to avoid some confusion and to show a clear difference in genetics Shantibaba took his Cup winning White Widow and renamed it Black Widow and his Cup winning White Rhino was renamed Medicine man.
The strains that Green House Seeds sells using the names of Shantibabas creations are knockoffs, just as any other breeders genetics that carry the same name but are genetically different.
The similarities between breeder lines do not go all that much farther than shared strain-names for genetically different strains, like is the case with most knockoffs of famous named strains. After the shared name strains the lines are no more similar than any other lines.