I hope this message finds you well. I am licensed attorney with over a decade of experience handling matters like the one before you.
You can do this but you will have to file a petition with a court of competent jurisdiction that this property conveyance should not happen and that your husband told you as much before he passed...but before the trust document could be amended.
If you simply do not convey the property, the sister in law could sue for property to be released to her, or in the alternative, for the fair market value of the property to be awarded to her. She could arguably get attorney fees tacked on as well as you she would argue that you breached your fiduciary duty
as the executor to the trust in not following it out.
Since this was your husband, and therefore would otherwise be your personal property, you have at least a good chance, if not a great chance, to have this portion of the estate amended by the court. However, to be completely above board, legally speaking, you are going to have it changed by the court - which means filing a petition and litigating the matter.
You may want to hire the attorney that drafted the trust to represent you in the matter. It is not something you would want to handle in court alone.
Let me know if you have any other questions.