Jacob married (or at least had sex with) Leah and Rachel, and God seemed to approve of the arrangement -- opening and closing wombs for them both.
Jacob said unto Laban, Give me my wife, for my days are fulfilled, that I may go in unto her. And Laban ... took Leah his daughter, and brought her to him; and he went in unto her. ...
And ... he gave him Rachel his daughter to wife also. Genesis 29:24-28
And God himself told a story about his own unhappy marriage with two sisters.
The word of the LORD came again unto me, saying, Son of man, there were two women, the daughters of one mother ... And the names of them were Aholah the elder, and Aholibah her sister: and they were mine, and they bare sons and daughters. Ezekiel 23:1-4
(This was, of course, just a parable or metaphor. But there is no indication in the story that God disapproved of this arrangement.)
God forbids a man to marry his wife's sister, while his wife is still alive.
Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time. Leviticus 18:18
