First thing to note, is the languages are completely different. Japan borrowed Chinese characters some time in the first millenium. At first they used the chinese pronunciations to try and spell out their native japanese words, with no regard for what the chinese characters actually meant. Later, They started using the chinese characters with their original meaning- but they also borrowed the chinese pronunciation, which greatly enhanced the vocabulary of japanese. However, it wasnt that simple- they also pronounced the characters with the native japanese words. This is why the sign for person can be pronounced 'nin' or 'jin' (i believe, but i could be wrong, that the modern chinese equivalent is 'ren'- some changes were made in pronunciation to accomodate chinese sounds to the japanese phonological inventory), but also 'hito' (the indigenous japanese word for 'person'. Therefore, they practically doubled their vocabulary and got a free writing system to boot, which was later adapted to create hiragana and katakana (the syllabaries) as well for specialised purposes. You can usually tell if a pronunciation of a kanji is chinese or japanese- if it is monosyllabic, it tends to be a chinese borrowing, if not, it is often the native japanese word.
None of this makes the languages related. It is simply a case of borrowing. It's like when people assume English is related to Latin because we have borrowed so many Latin words.
Hope this helps
