Do not be sexually intimate with the sister of one’s wife during the wife’s lifetime
“Do not take a woman in addition to her sister to make them rivals, to uncover her nakedness while with the other, in her lifetime” Vayikra 18:18
From when someone marries a woman her sister becomes prohibited to him.
Whether a sister from the same father or the same mother, even though immoral relations.
If he is sexually intimate with her sister while his wife is alive, even if they are divorced, they are punished with spiritual severance.
If done unwittingly a sin offering is required.
This applies in all place sat all times.