English Translation
Your big sis will show you.
I’m sure 95% of Japanese native speakers translate お姉さんが教えてあげる as “hey naughty boy, I’ll teach you something good, hehehe” in a sexual context. お姉さん is not “your big sister” but talking about myself who has many experiences in that matter. And 教えて「あげる」is used when the subject is myself, not a third person.
“Your big sis will show you” should be translated as:
あなたのお姉さんが教えてくれるでしょう。