"Vím o tom od sestřina přítele."

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

[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.

[JohnBraga1]

If it was a case of sisters rather than a single sister, the English would be sisters' (as opposed to sister's). Would the Czech still be sestřina?

[AgnusOinas]

The possessive adjective ("sestřin") only works for singular entities.

To say "sisters' friend", we have to go back to the good old genitive: "přítel sester" - literally "friend of sisters". (The genitive is an option in singular, i.e. "sestřin přítel" = "přítel sestry")

[JohnBraga1]

Thank you!