
What’s the purpose of this? There’s not even a story line

Read the title lol
This is literally just a pornhwa my friend
Although it has no plot you can tell it's written by a woman. No huge tits on every female in the story and no ugly males just because they're not important/the main character. Neither of those paired with bad writing covered up sex as the answer for everything.
With this we literally just follow a woman's pov who likes sex and doing daring explicit things

I’m super confused by this story and the female lead is annoying af.

The manga isn't very detailed. If you want to know more, you need to read the novel. It's a lot better and more tragic.
https://www.novelupdates.com/series/my-fiance-is-in-love-with-my-little-sister/
No backbone what's so ever