.

The father could not have children, so his brother the uncle donated his sperms.
After the father found out, there were twins, he wanted to give one to his brother as thanks for the help. But the uncle could not bear to separate the twins, so they grew up together.
Since the uncle should have adopted one of the twins, they are biologically twin brothers, but "lawfully" cousins.

Good explanation. Thanks from me.ヾ(☆▽☆)

Thanks to you i understand that part now.

I haven’t read this manga yet but I know how these stories go and since cousins aren’t considered incest in Asia, will the twin brother go after him or something?