I believe Toriko and Starjun are the children of Froese and Acacia. Joie is the reincarnated Froese with a different soul. Froese was pregnant with a child(ren) when she died. Acacia and Joie both believe that Joie is that unborn child; however, Midora refutes this by explaining, "Joie is the reincarnation of Froese herself... not the grown up form of whatever was in her womb"Â so never specifically says a boy, girl or twins. This leads Acacia to believe the child was never born, but Midora does not respond. If Joie is the real body of Froese and reborn, wouldn't Joie have noticed being pregnant? Where did the child go? So I think it is safe to assume the child also survived. I think when the children were born Midora adopted Starjun and abandoned Toriko(why Torikos origin starts in the wild), and I assume that Starjun is the older child and probably showed more talent/potential to Midora which is why Starjun won in their fight. I think this also explains Toriko and Starjuns potential, and appetite demons.
It is also makes sense why Starjun keeps asking Joie if he knows whos children they are. Maybe Starjun already knows this which is why he doesnt kill Joie. Also I dont see any point in Starjun asking Joie this question if Toriko and Starjun were the children of Midora, why would Joie care if they are Midoras children? They are his enemy, whereas maybe this question would change Joie's mind?