Anitachi no Aisarete, Korosarete.

After Mahiru lost her parents in an accident, she lived with her three step-brothers who loved her dearly, even though they weren't blood-related. Until the death of her estranged grandfather, who left a will saying that whoever married or killed Mahiru first would inherit his entire estate, worth 15-billion yen. They thought their sibling bond was unbreakable, until the shocking announcement of the will caused them to waver!? The ultimate love mystery! (Source: Flower Comics, translated)