Condolences, tanith 
I think it's understandable that your sister is reluctant to go, it's a long drive and she is 81. Driving really is not a pleasure now, particularly on motorways.
It would be perfectly all right, imo, for her to telephone or write to your sister-in-law and explain, which I am sure she will do.
You will be there to represent that side of the family too.
Updating bathroom with a walk-in shower unit.
