They should just live out at sea. The international laws of the ocean are very loose.
While there's no one out there to enforce any laws, you are always bound by the laws of your homeland wherever you are. If it's found out that you did something illegal out at sea you can be arrested once you're back inside your country's borders.
I was just asking the others...What?!Don't give up, Miho!Onee-chan...MahoxShihoxMiho mangaMiho, what's wrong?!Onee-chanAnd they told me sisters can't get married!There has to be a way...If sisters can't get married...Then does that mean a parent and their child can't either...?!