Unfortunately / [infml] I'm afraid I can't come to see you as arranged at Christmas / we can't accept your invitation for the 9th owing to unforeseen circumstances. My mother is seriously ill. / I have broken my leg. / I have to go away on urgent business.
[fml] Please tell your sister from me how sorry I am.
lub
Please, convey my apologies to your sister.
It is a great disappointment to me. / I shall really miss seeing you.

[fml] - formalnie [infml] - nieformalnie