I think Juliet did leave her sister behind for two reasons.
1-because her sister wanted to her go and further the research that had helped her to conceive
2-because she was promised she could be back in six months well before the birth.

As for why she would impregnate a woman who was very sick with cancer...that could be a combination of things. First, I am sure it is her nature to push the envelope, which is how many great discoveries are made. But more than that it was something her sister had always wanted. Having a baby to live for would give her renewed hope and the strength to fight her illness. In the event that she did not survive the cancer, she would find comfort in being able to leave something of herself behind...her baby. Of course she knew that in the event she did not live to raise her child Juliet would.