The poem by W. H. Auden that they use in 4 Weddings and a Funeral expresses grief and loss very well but is really more about the loss of a lover. You could select bits or modify it.
Stop all the clocks, cut off the telephone,
Prevent the dog from barking with a juicy bone,
Silence the pianos and with muffled drum
Bring out the coffin, let the mourners come.
Let aeroplanes circle moaning overhead
Scribbling on the sky the message He Is Dead,
Put crepe bows round the white necks of the public doves,
Let the traffic policemen wear black cotton gloves.
He was my North, my South, my East and West,
My working week and my Sunday rest,
My noon, my midnight, my talk, my song;
I thought that love would last for ever: I was wrong.
The stars are not wanted now: put out every one;
Pack up the moon and dismantle the sun;
Pour away the ocean and sweep up the wood.
For nothing now can ever come to any good.
My own all time favourite poem might do the trick
Lodged
“The rain to the wind said,
You push and I'll pelt.'
They so smote the garden bed
That the flowers actually knelt,
And lay lodged--though not dead.
I know how the flowers felt.”
― Robert Frost
I'm really sorry to hear about your sister, DrZ.
There are others here who have an extensive knowledge of literature (though it will probably mostly be English). Together we will find you something appropriate.
If you want it to be from the heart you might need to write out your own grief.
