Oh now you are asking yourself whether raping and gaslighting your grief-stricken sister-in-law is wrong. Well, the rice has been cooked to porridge and there is no way to turn it back might as well made the most of it even if it leads you to your doom. At least she isn't sad all the time.
She wants a child with her brother in law, she probably fears being left behind again. Once she gets pregnant all her worries would be gone and she should return to what she was before everything fell apart for her. Brother in law should have taken it slowly though and not force his way into her in the first place if he didnt want this outcome.