Ive just walked in on my Little Brother having sex should i discuss things with him?

[??CANDY??]

Okay im 16 and i walked into my little brothers room to clean up and didnt know he was in, thought he was out with his friends and i saw him having sex with his girl friend Hes 15 btw GF same age, he said i could not tell mum about it so i havnt should i sit him down and talk about things , it is alittle awkward between us now ...