For ED, does sibling legacy help – even if your sister is currently attending the university?
If the sibling is doing well at NU and if that sibling can put in a word for you or getting you in touch with the ‘powers to be’ 
It might or might not it really depends on your overall profile. If your stats are within the general admission parameters that its an additional plus for maybe why admissions would choose you over another student with similar stats. I heard applying this year that legacy stuff only really mattered ED (but I know they ask you on the Common App for both ED and RD), and usually parent legacy is considered much more valuable. I got in this year ED with 3 sibling legacies that may have helped somewhat (I’m the youngest of 4 so I kind of played the “please don’t make me the family embarrassment” card), but I also had other good factors going for me so I really can’t vouch for how much sibling legacy “helped” my case for admittance.