Im thinking big sis breaking her lil bro innocence since kids, starting small with touches, kisses on the cheek, sitting in each other lap. Then big sis start to molest lil bro body, saying things like "your body felt so nice!" making lil bro fidgety and blushy all over. Kisses eventually going deep as they explore into each other mouth, big sis taking the lead, teaching him here and there. Though the table turned as lil bro grows up being infatuated with his big sis. She's the one who teach him everything, she's the one who shows him to do it, she's his reason. So it doesn't take much as lil bro took the charge. Rough missionary with big sis arms pinned above her head as lil bro pumped his cock inside her cunny over and over again, got a wide grin on his face, placing kisses all over her teary face, load a big dump into her womb, he's watching in satisfaction as his big sis twitch and tremble under him, keeping his cock inside and making sure not a single drop was wasted. Popping his own cherry and big sis just had to be his first time. He never felt so complete after this.