Why do you have to reply? So they know how many "family" seats to save at the funeral, and how many corsages to order? Sigh.
They put you in an awkward position by not informing you in a timely manner of your uncle's passing. Now they expect an RSVP? Sigh.
OK, say "I regret I'll be unable to attend." If you could make it even shorter than that, it would be even better. Sigh.