"whom" is OK?
She looks so young I took her for your sister whom I saw at the party.

I want to add extra knowledge about 'her' saying I saw her at the party.

Question

"whom" is OK?
She looks so young I took her for your sister whom I saw at the party.

I want to add extra knowledge about 'her' saying I saw her at the party.

Answer

Yes well done. Add a comma before "whom" because this clause is extra information. "who" would also be acceptable. "whom" is formal.

Answer

Hi Omer,
Whom is just fine.  Who and whom are not easy to work with.  Well done!
Have a great day,
Stephen