all one of the sisters has to do is tell the courts that she self-identifies as the other sister (IE She believes, in her sick twisted adulterous mind, that she is her sister)- that way it will be a normal two person marriage because now anyone can identify as anything or anyone they choose and society has no right to question it apparently
The Man marries one sister and she marries her sister
Since the man is married to only one woman, it is legal. A sister wedding a sister might be incesteous but there is thus no bigamy