Hello Donald,

The Maughold parish reg has:
John [son of] John Kermeen/ Isabell [bapt] Parish Church 28 May 1741
Cath John Kermeen/ Issabel Parish Church 28 Nov 1742
James John Kermeen/ Isabel Parish Church 4 Dec 1744
Thomas John Kermeen/ Isabel Parish Church 7 Mar 1750/51
Isabel John Kermeen/ Isabell Callow Parish Church 14 Oct 1753
Jane John Kirmeen/ Isabel Calow Parish Church 22 Oct 1758
Mary John Kirmeen/ Isabel Calow Parish Church 12 Apr 1762

No sign of Ann or Joney apart from the 1751 burial which Clive noted.

Margaret William Kermeen/ Isabell Parish Church 17 May 1741
William William Kermeen/ Isabel Parish Church 13 Nov 1743
Anne William Kermeen/ Isabel Parish Church 5 Mar 1748/49.
Thomas William Kermeen/ Isabel Parish Church 6 Apr 1749.
Thomas Son of Wm KIRMEEN (Ballig) buried Feb 26 1759?

The last two can't be brother and sister, if bapt near the birth date.

N.B. There were other Isabels married to William Kermeens who could be the above Isabel:
e.g. William son of William Kermeen/ Isabel Kennish bapt Parish Church 15 Mar 1753. Private baptism.
Also children of William Kermeen and Isabel Craine.

You mentioned Tom Kermen, who had a lot of research done on these Kermeen families, much of it in land records by Constance Radcliffe. Have you been in touch with him in recent years? I tried to track him down a couple of years ago without success, and would love to make contact again.

Sue