Both have scheming families with an evil stepmom and sibling who gets in the way of childhood sweethearts. The star crossed lovers are separated by studying abroad, amnesia, car accidents, and terminal illness. Shin Hyun Joon stars in both dramas as the second male lead who wants the amnesiac main character's memories to stay forgotten. Their love is selfish but you can't help but feel a bit sympathetic. Choi Ji Woo in Stairway to Heaven and Han Ji Min in Cain and Abel are phenomenal at crying scenes that make your heart ache for the female leads.

Stairway to Heaven has a female main character who's in a love triangle between the childhood sweetheart male lead and the second male lead who's her non-blood-related brother. The stepsister played by Kim Tae Hee has extremely evil-looking acting with bulging eyes.
Cain and Abel has a male main character who's in a love triangle between the childhood sweetheart female lead and the other female lead who's his non-blood-related fake cousin. The adopted brother played by Shin Hyun Joon has extremely evil-looking acting with hawk-like glares.

Both dramas have mature storylines with realistic, selfish characters who don't consider love as the only factor in a relationship. The characters in both dramas follow similar archetypes - the rich girl who's cold on the outside, the rich guy who wants to be loved, the poor guy who wants to rise up, and the poor girl who has the attention of both guys. Everyone works at the same place and tensions rise. In both dramas, the male lead is a jerk with a heart - although whether he's the rich one or the poor one is different in WHIB compared to QSS.

So Ji Sub as an angsty melodrama male lead who meets a poor but plucky girl working as a tour guide in another country. Things get complicated when she arrives in Seoul and they are reunited. Lots of tears from the female lead.

A major difference is that Cain and Abel has firmly established couples by the 2/3 mark that you will root for while What Happened in Bali keeps the love square unresolved until the very end.

In both dramas, the main character is a kind and caring doctor who's deeply in love with his girlfriend. Through a series of tragic events, he is betrayed and loses his identity as an upstanding and respectable man. The story follows his revenge with a family divided by power struggles over the ownership of a company. Both dramas are classic melodramas with tropes like love squares, amnesia and illness.

Both have a classic melodrama story about love triangles, revenge, and complicated relationships between brothers.