The more convoluted and messy it is the better ;) 

I would recommend you to check out the dramas that are named in the article written by @Cho Na a few days ago  Step sibling romance 

Pretty much all types are named in there ;) 

In addition:

  • Autumn in My Heart - lived until their teen years as siblings but separated after finding out  FL was switched at birth, meet again as adults
  • Tree of Heaven
  • Golden Fish 
  • Triple
  • Marigold in 4 minutes - this one is not really messy, and more wholesome with people rooting for the couple