My father passed away almost 30 years ago at age 65. Prior to marrying my mother, he married another woman and had a daughter from her, then divorced around 1947-48..

He never really spoke about that marriage, but we know it occurred at approximately 1939. (see update 2) We suppose the marriage was in NYC district, which he lived his whole life.

We're anxious to find out who is (or was) our sister.

P.S. our family name is very distinctive, might that help to find entries in the various archives?

What actions can I take to find out about his marriage?

Not that I have much, but I'll provide more specific info on demand.

Oops, forgot to mention, I don't live in the USA, I want to do it all from abroad.

Update

After a year of work, I managed to get the marriage certificate and I now know who my father's wife was. The marriage took place on 1943.
Now how can I get down and find my sister? The marriage certificate contains no SSNs. Where can I find her (mother's SSN, or what's her new name if she got married again)?

I tried to trace them on some online searching websites like Ancestry, FamilySearch and others.

• Are there any official or non-official documents I'm eligible to obtain as my father's son, perhaps via hiring a professional genealogist or lawyer, towards finding my sister?
• Any other ways I can find out who my sister (or her kids), is and whether she's alive or not?
• What are other options I can check?

P.S. I believe it was the first marriage for both my father and the bride at the time.