Have you searched on FamilySearch.org? Most of the death certificates from NYC are indexed AND partly extracted on family search. You can get a free account on it. If you are successful in finding the record of her death, you can download a copy of the record once the LDS Family History Centers are again open to the public. With regard to your father whose name you did not list, there were two or more Kaminer male births in NYC according to Familysearch.
Another place to search is via Ancestry.com which is available at many public libraries. Ancestry will allow access free at libraries until the end of 2021. However, you can’t download a copy of the death certificate there.
You did not provide the name of the person whose data you are seeking, but since it was a sister to your mother, the name probably was not Kaminer. You implied that this may have been an event that occurred outside of marriage, so you would be searching for someone with your mother’s maiden name.
Suggest you start your searches via Ancestry and FamilySearch.org. The latter you can do from your home computer.
Researching: KOLBRENER (LezaJsk); SCHWARTZ (Glogow Malapolski); ZINDEBAND (Minsk); LIFSHITZ (St Petersburg); JABELOW (Minsk); BOHRER (Lezajsk); POLLACK (Minsk); BLUMSTEIN/BLUMSZTEJN (Goworowo); FRETER/FRATER (Czyzewo)