I am not expert in Latin.

As far as I know all Indoeuropean (AKA Arian) languages like Slavic (including Russian), Germanic (including English) and Romance (including Latin) have common protolanguage, which was more complicated in grammar than any modern language of this group. All languages was simplified step by step as a general trend so Latin is more simple than protolanguage and modern Romance languages are more simple than Latin. Russian and English are from other branches of the same tree but with the same simplifying process. Now the most close to the protolanguage modern language is assumed to be Lithuanian. I do not know what about Russian and English in comparison to Latin. There is no way to calculate similarity of grammar in one number AFAIK.