Keiichi's parents threw him out of their house when they found their son was gay. He still was a teenager at that time, so I suppose he has no contact with his parents anymore. They probably don't know Keiichi adopted the twins. And, anyway, it's not their grandkids as Ren and Takumi aren't Keiichi's real sons.
But still it's his sister's sons
they are not. they are her ex husband's kids (not with her, but with other woman) that he left when she died (they made it so complicated lol)
I wonder what will his parents think when they find out their son is ducking with their two grandkids