At my church we have an online prayer list that circulates throughout the members of the church. I just added your mother in law, you and your family to it. Lots of people say a prayer when it gets updated.
We beseech Thee, O Lord,
in Thy mercy,
to have pity on the soul of Thy handmaid;
do Thou, Who hast freed her
from the perils of this mortal life,
restore to her the portion of everlasting salvation.
Through Christ our Lord,
Amen