If you watch the last episode of the show, you see John Churchmouse writing in his book at the end. Mattimeo then comes in, and John shoos him away. Right after that, he says "John Churchmouse abbey recorder, father of Cornflower, grandfather of Mattimeo, son of Matthias, warrior of Redwall."

But, in the original story, Cornflower's father was named Mr. Fieldmouse. So, if John was the father of Cornflower, that would mean in the show since mattimeo married Ress, and she is the daughter of John Churchmouse, then Mattimeo married his aunt.