I agree, this is very odd.
The will was transcribed by Brian Lawson in 2002 and it says the Court "hath decreed her three children, namely William, Thomas and Jane Killip, joint administrators ..."

I don't know who Jane is. One thought is that perhaps "Jane" should be "John", but he/she is listed last when I would have expected John would be first (though names aren't necessarily in birth order in an Admin, but tend to be other than sometimes males are first).

The IGI only lists 3 children for a William Killip (no mothers listed in Lonan then) in the time frame 1718 to 1790: John 1758, William 1762, Thomas 1765. The only Jane Killip was in Andreas in 1763, and too close timeframe to William's 1762.

The father William outlived Isabella and is probably the 1788 Lonan will. We need to see his will before drawing any more conclusions.