I also was puzzled with the idea that a half brother/sister combo would marry, so I've dug a little deeper. I think I have it figured out now, and we can throw out the slightly-too-close family knit we all were concerned about. Whew!

Re: Margaret CRAINE (m Wm KEWLEY 1851):

Based on the 1861 and 1871 census, Margaret (CRAINE) wife of William KEWLEY was born in Onchan about 1823-5. There is a baptism in Onchan of Margaret CRAINE 23 Nov 1823 to Robert CRAINE & Martha CHRISTIAN that fits quite nicely.

I looked at the children of William and Margaret to see if they used parents names. First son is William Robert. A daughter is Emily Christian. Though I would have liked to have seen a Martha to be more certain, these aren't bad clues.

Robt CRAINE m Martha CHRISTIAN 8 Feb 1823 Onchan. Children: Margaret 1823, Caesar 1825, William 1828, Robert 1830, Jane 1832. In 1841 they are at Ballig, Onchan: Robert Ag Lab 40, Martha 45, Robert 12, Jane 9. In 1851 they are at Little Mill, Onchan: Robert 50 Ag Lab b Douglas and Martha 60 b Onchan. In 1861 in Onchan there is a Martha "CAINE" Widow 72 living alone with a blank in place of birth. Burial Martha CRAINE 75 Onchan 12 Mar 1862 (Brian Lawson's burial index).

I looked around to see if I could find Margaret CRAINE (b ca 1823) in 1851 and 1841 census's. In 1851, I think she will be the Margaret CRAINE age 27 House Servant on Victoria Road Onchan at the home of a Frederick B WALKER. In 1841 I think she will be the Margt CRAIN 17 F.S. on Sand St. home of Robt PARK.

Likely burial for Margaret (from Brian Lawson's burial index): KEWLEY, Margaret 55 Bra 15 Oct 1879.

Now, over to William KEWLEY:

I relooked at the 1871 census to see if I'd made a mistake, but it definitely says that Lucy QUAY is "mother in law". But I tend to agree with Frances that the 1827 Braddan baptism of Wm CRAINE illeg son of Edwd KEWLEY and Lucy CRAINE ( marries John QUAY in 1829) is actually William QUAY age 15 in the 1841 census and then William KEWLEY subsequently. So, if this is all true, the 1871 census for Lucy QUAY age 69 should say "Mother". If we accept that, everything else falls into place.

Lucy CRAINE bp 29 Sep 1799 Braddan d/o Robt CRAINE & Margt QUARK.
Robert CRAINE m Margaret QUARK 21 or 27 Dec 1791 Braddan. Children: Caesar 1792 or 1793, Moore and Robert (twins?) 1794, Lucy 1795 (died?), Catherine 1796, Anne 1798, Lucy 1799, Mary 1803.

The appearance of "Caesar" in both Craine sets is interesting. There's a hint of a potential relationship.

Edward KEWLEY (who liaisoned with Lucy Craine to produce our William in 1827) could be one of several. There were about 5 baptized in a 5 year period around 1800. An Edward KEWLEY married Catharine McCLURE 20 Nov 1830 in Braddan and another one married Deborah QUAYLE 30 Aug 1838 in Braddan. Both are in 1851 census and approx the same age born ca 1801-1804.

I hope you can make sense of all of this.