I have watched some dramas with such plot like "Something in the rain" and "What's up fox?" which were both noona romance dramas im looking for such dramas where FL & ML's sister are BFF 

Not BFF per se, but  ...

  • Koi wa Tsuzuku yo Dokomademo - new friends that really hit it off so awesomely and become very very good friends
  • Le Coup De Foudre - the FL's Brother is the love interest and forecer crush of her BFF
  • Forever Love - FL's brother-like-cousin, is the first love/crush of FL's BFF 
  • Marry Me, or Not? - the two girls are ex-BFFs, had a misunderstanding that lead to a major falling out and became arch enemies...?