MI means memorial inscription, ie the wording on the headstone, if one was erected. Since Isabella's husband was a baker, he probably did erect a stone for her ( lots of people couldn't afford to )

Donna Douglass has bought the books which list MIs in various graveyards on the IOM, and she's kind enough to do "look ups".

In this case, the MI might mention Isabella's maiden name.

KEWLEY, Isabella 43 Bra 8 Jan 1899