We sincerely hope that you won't have to worry about postponing or cancelling your wedding, but just in case here are some suggestions of the appropriate wording

Family Bereavement

'Owing to the recent death of Mr Robert Jones, Mr and Mrs Jack Smith regret that they are obliged to cancel the invitations to the marriage of their daughter Chloe to Mr Joshua Jones.'

For a Postponement

Owing to the illness of Mr Robert Jones, Mr and Mrs Jack Smith regret that they are obliged to postpone the invitations to the marriage of their daughter Chloe at St Mark's Church, Darling Point, Sydney. from Saturday 22nd January to 1.00 pm on Saturday 21st February 2009.

Broken Engagement

Mr and Mrs Jack Smith announce that the marriage of their daughter Chloe to Mr Joshua Jones, which was arranged for 22nd January, will now not take place.