Maybe they're half-brothers. That would explain the different surnames.

Possible, but I have my doubts. I think they're full brothers.

Or because they were fellow associates the term 'brother' is used because the Lin Kuei were tight. After all Bi-Han (Noob Saibot) states they share blood but they are not brothers. Whether that's because Bi-Han's mentality has altered and is messed up who knows. I thought Kuai Liang and Bi-Han don't have issues where as Sub-Zero and Noob Saibot might.

I think they're brothers because Noob says to Sub-Zero "You disappoint me" then Sub-Zero says "The feeling is mutual" and then Noob says, "Mother would be so proud."

I'm going by the way Noob Saibot was in MK2011 and before (sinister). The soulnado encounter changed him, so in MK11 i think he's more wise learning to have a slight remorse, going by the "mother would be proud" speech.

I'm pretty sure Noob was being sarcastic when he said, "Mother would be so proud". As far as I read both Sub-Zero's are brothers and lived in America before returning to China. The Lin Kuei was known for abducting children, but we don't know if this includes both Sub-Zeroes. In any case, they both joined the clan one way or another.

Most Lin Kuei are kidnapped as children around the world and raised to be assassins. They are usually street urchins with no future (Smoke), but I'm certain Bi-Han and Kui Liang were found together and understood to be actual brothers. That's why they have so many different ethnicities in a Chinese assassin clan but the Shirai Ryu are just Japanese ninja.

Btw, I may be wrong but I'm positive I'm right.