Blighted Ovum is death looking version of the Gemini boss. He was added to The binding of Isaac on its DLC wrath of the lamb. He has more change of spawning in boss room as boss If you are killed Gemini on same run.
The Older sibling appears as mangled and zombie looking humanoid that has part of skull of his (possible) eaten by something. He leaves trail of blood behind him that will hurt Isaac if he touches it. Unlike Gemini, he will countinue chaseing the player all the time (Meaning he dosen't take any kinds of stops while doing that)
The other sibling is pretty much passive, unless Isaac walks in his line of sight, in that case, will shot brimstone like laser. He dosen't move so much from his spawn point.
Duke of Flies | Gemini | Monstro | Larry Jr. | Famine | Steven | Widow | Pin | Blighted Ovum | Gurdy Jr. | Fistula | Gurdy | Peep | Chub | Pestilence) | C.H.A.D. | The Carrion Queen | The Husk | The Hollow | The Wretched | Monstro II | Loki | Gish | Mom | Mask of Infamy | Daddy Long Legs | Triachnid | The Bloat | War | Scolex | Blastocyst | Conquest | Teratoma | Lokii | Satan | The Fallen | Headless Horseman | Krampus |