From Wiki:Isoroku không phải true end nên thằng Rangi đâu có được coi là official :123: chỉ có Dark Rance với Reset thôi mà
However, There is speculation that she had a child with Rance even in the True route, as in Rance Quest, there is a mention of him, as if Rance was the father of Rangi. Possible theories were that the charms Isoroku had or her constant prayer contained some magic to negate the contraception spell, or that when Sill was frozen, the contraception spell got negated and he 'had' Isoroku one more time before he left. Also, Rance once pretended to be a local god so it could be divine punishment of sorts, should the gods exist in the Continent.
It was also implied in the Sengoku Rance Epilogue that Yamamoto Isoroku became pregnant after the events of Sengoku Rance True Route. Natori noticed this first.