how are they not blood related if they're his sister's sons?

I think his sister is the second wife, and the first wife is the mother of the twins, so they are not related ig ╮(￣▽￣)╭

It's his sister' step son
The real mother is the first wife not his sister', read it again

I thought this was gonna say "how are they not blood related if they're twins" and laughed sm at the idea of a manga trying to pull that somehow