A romance setup where the love interests are tied together by having one of them be Best Friends with the other's sibling. This dynamic has both familiarity and potential conflict built in, so it's a popular choice for the romance arc.

Generally, this allows the relationship to be familiar, while still somewhat taboo. The love interest is easily introduced, frequently around, and might even develop an affectionate relationship with the protagonist as well, but one or both are reluctant to progress the relationship. The "drama" part comes from assuming, correctly or not, that the "best friend" will disapprove of the relationship — either