Here's what I suggest, based on my research to tell apart persons from NYC with the same name, similar year of arrival, similar age, spouse's name -
1. Get a copy of the image of the marriage and death records. These will have the address of residence for the persons of interest. These can be matched (or not) to addresses from census records, draft cards, naturalization papers, to tell people with the same name apart.
How to get the images - There is a facebook group called New York City Genealogy. Every day, the admin named Melanie Curtis Egan starts a new post, for those who want the images of birth/death/marriage records in NYC, based on the FamilySearch page for each record. There are persons in that group who know how to get the images from their local Family History Center via wifi from the Center parking lots. The post from the admin lists what info they need from a requestor to do this.
2. For a death record, the image will show the cemetery where the person was buried; this should match what you know from gravestones or obituaries of the deceased.
3. As for your two situations above -
A. What other evidence to you have from records that Mary Gordon and Jennie Gordon are sisters? Do their ages closely match what you have from census records, etc?
B. Marriage and death records for Isaac Jaffe - is there evidence that these are the same Isaac Jaffe? Do their ages in the records match up? Do their addresses (obtained from the record images) match up?
The challenge with NYC vital records is that so many early immigrants had relatively common Jewish surnames, and took/had first names from a limited pool: Ben, Morris, Isadore, Isaac, Abraham, Sam, Jacob, Irving, Charles, Joseph, etc, etc, etc. And for women - Sarah, Rose, Esther, Minnie, Fannie, Tillie, Bessie, Lina, Sylvia, Ada, Ida, Pearl, etc, etc, etc. All immigrating to NYC in the early 1900s, most in their 20s. How many of us have family trees filled with these first names, over and over again?
A case in point - I helped a friend look into his Citron family in NYC. An uncommon Jewish surname - and yet I found two Ben Citrons, both living in the Bronx, both similar ages. One married to a Fannie, another to a Tillie. Figured out who was who based on the image of a death record and ages of first born child. And then I found two couples named Sam and Lena Citron, with both women born the same year! Took a naturalization paper to figure out who was who, plus comparing addresses from census records and draft cards, as well as occupations of the men.
So, to tease things out, get more records on the individuals you are trying to trace, and go from there.
Lock/Lak/Lok and Kalon in Zagare/Joniskis, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine in Trenton, New Jersey and Lida/Minsk gub., Belarus