Rebecca is singular, therefore girl is singular.
The following is grammatical:
Rebecca is more beautiful than any other girls I know.*
My friend shreds harder than any other girls I know.
And, If you change the wording, then the following is acceptable:
Rebecca is more beautiful than any of the other girls.
Thus, I am not convinced that the following is ungrammatical:
Rebecca is more beautiful than any other girls.
In addition, you can use plurals in comparatives:
“Jersey girls have this inner glow that makes them more beautiful than any other girls.”
Jersey girls is a plural subject; therefore the plural girls is compared to the plural (any) other girls.
You probably will not run into Girls (plural) are more beautiful than any other girl (singular).