Don't let go of her, always keep her in prayer and ask God to give you the moment, the opportunity to breakthrough. But be willing to let go and allow that breakthrough to come from someone else, you may pray but someone else may deliver. I will pray that God will give you direction and wisdom as you labour in love for your sister-in-law.
Dear Bill & Carol, the loss of a loved one can sometimes be very hard to bear but always remember God understands. His only Son died on Calvary so he knows the pain, He will comfort you in your time of need and your brothers in the Lord will remember you in prayer.