which one to use in this sentence, que o lo que?

Question

Should I use que or lo que in the translation for this sentence: "I didn't know they already knew our nephew.", which one is correct and why.

1. Yo no sabía que ellos ya conocían a nuestro sobrino.

2. Yo no sabía lo que ellos ya conocían a nuestro sobrino.

What it be if the sentence is "I didn't know what they already knew our nephew."

Thank you.

Accepted Answer

Hola Asuka
"I didn't know they already knew our nephew."
You need number 1: Yo no sabía que ellos ya conocían a nuestro sobrino. 
I didn't know [that] they already knew our nephew.
I don't quite understand what you mean in your last sentence, because the English doesn't make sense: 
"I didn't know what ?? they already knew our nephew."
Here is a Kwiziq lesson on "lo que" that will help you understand better and you will see how it doesn't apply here. I hope it helps.
Lesson on "lo que"
Saludos
Inma

Answer

Thank you for your reply. I meant to say "I didn't know what they already knew about  our nephew."  "lo que" o "que" in that?

Answer

Hi Asuka
In that case then you need "lo que":
Yo no sabía lo que ellos ya sabían de mi sobrino. 
Inma