If antecedent is un-ambigious as is the case here uses of which is correct, as which can not refer to a person u need who for that

aditya1984 wrote:Emily Dikinson's letters to Susan Huntington Dikinson were written over a period beginning a few years before Susan's marriage to Emily's Brother and ending shortly before Emily's death in1886, outnumbering her letters to anyone else.

Ans as per OG:
Dikinson, which were written over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in1886, outnumber

As per modifier rule a noun modifier must touch it, but here "which" is not touching "letters".Can any body explain why still this is correct answer?

The clause "which were written over a period beginning a few years before Susan's marriage to Emily's brother and ending shortly before Emily's death in1886" is a noun modifier of "letters". The catch is, the modifying clause is NOT essential to identifying the letters. The beginning of the sentence tells you exactly which letters are discussed...Emily Dickenson's letters.

You could say THESE letters. THESE is an essential modifier to describe exactly the letters in question. Therefore, the modifier, separated by a comma, does not need not touch the noun being modified, because it is not essential.

To change the sentence to make the modifier essential you could say, "the letters that were written over this period....."

I hope that makes sense.