MeruPuri: Märchen☆Prince, Yoru made Matenai. Recommendations
MeruPuri is a lot better compared to Yoru Mad Matenai, but they have similar concept of the younger [brother-like] boy falling in love with the supposed-older sister. it's somewhat taboo, but it isn't because neither of their relationships are actually blood-related. Meru Puri has magical themes embedded into the story, whereas Yoru Made Matenai is just a story about love and has some sexual scenes. however, i have to say that both of the male characters are super cute!