Given that first bubble has both "sex" ("セックス") and "condom" ("ゴム") in it, I think I made a good stab at what was being said by onii-san, but obviously would still be better for someone with more than a gradeschool-level understanding of Japanese to check what was actually said.
Oh, and by the way.
When you two have sex, don't forget to wear a condom.This isn't an ero-manga, you know...I know that...WAIT, AS IF!!!yellingDamnit, I shouldn't have mentioned it to my brother...There's no way I could do that with my classmate...But, if I were to ever ask her to pose as a model...Master...♥splash!SE-!?