"Vím o tom od sestřina přítele."

Translation: I know about it from my sister's friend.

[aososam]

I know about it from a friend of my sister - is it wrong?

[igamenir]

That would be "Vím o tom od přítele mé sestry."

[Sa-Matra]

I know it from my sister's friend - why not this?

[VladaFu]

That is just "vím to", not "vím o tom". The difference is subtle but I think it exists even in English.