How to refer to a deceased widow?

I’m thinking you may mean “refer to” rather than “address”, unless you’re really writing as if you’re speaking to her. But I may just be unaware of the tradition being followed.

Formally you’d use “the late” followed by her name as it was when she passed: I think you’d need to say “the late Mrs. Jaqueline Kennedy Onassis” even if you were a Kennedy. You could omit Kennedy or include her maiden name, if you wish. Basically use her name as her preference would have been, the day she died.

You can also say “my late grandmother”. I don’t think there’s a case where you’d use late just by itself before the name, though there is the idiom “John Smith, late of this parish” used in church to refer to someone who recently died.

Widows have traditionally most commonly kept “Mrs.” with their last married surname, but of course you’d use her actual preference if you know it. If she used “Ms.” or was a Dr. then you’d use that. On a formal invitation you would use the honorific.