Here’s a ritual for a deceased loved one you should adapt to your current situation.
Make sure to burn some cleansing incense (lavender, sage, rosemary) and try to feel the energy to identify if it is your mother-in-law.
In case you haven’t said a prayer for her spirit to pass on, here are a couple options since you mentioned that she was very Christian:
“May you rest in the arms of the Lord who formed you from the dust of the earth.
May holy Mary, the angels, and all the saints welcome you now that you have gone forth from this life.
May Christ who was crucified for you, bring you freedom and peace.
May Christ who died for you admit you into his garden of paradise.”
“Run the fields of Summerland
And frolic in the sun.
You’ve earned the right of freedom;
Your work on Earth is done.
Tho’ tears for you may fall like rain,
They are tears of joy mixed in with pain.
My love for you will never end.
Enjoy your time in Summerland, my faithful loving friend.”