@placemats
State of New York Death Records:
https://www.health.ny.gov/vital_records/death.htm
City of New York Death Records
https://www.nyc.gov/site/doh/services/birth-death-records-death.page
State of New Jersey Vital Records (including death records)
https://www.nj.gov/health/vital/
These are the portals to order records. I don't know if they'll take a 'foreign' address for delivery and I'd assume you'd have to be able to pay in US dollars. It's been too long since I was involved in obtaining vital records so I don't know if they supply records via email. They didn't when I was still working.
Many US Records Offices have restrictions on who is entitled to receive copies of records, especially for 'recent' events. Laws from state to state may vary.
Most of them will also require identifying information for your sister, which I assume you'll know. But they also normally require a date of death, or at least an estimate.
IIRC they'll search a range of dates, but not a 'broad' range. CAVEAT: I worked for a US Govt agency and our searches were for official purposes. A date range search may be something not available to the general public.
Good luck