What Should I Do About the Junior Sister I Raised Turning into a Monster?

I'm just a player who transmigrated into a game. I only happen to know the entire plot.

I was planning to raise my junior sister, then cling to her for support while we slayed demons and monsters.

But why...

"Senior Brother! You promised that once we reclaimed Yan Yun, you'd teach me what a 'wedding' is!"

"You... How are you anything like a gentleman? You're shameless and lewd! You have to tell me about this stuff next time!"

Did I do something wrong?