Turns out she is not Ann Jane Cain after all.

Ann Cain first married John Mylchreest 21 Sep 1871 Malew and they had a child Humphrey Stanley Mylchreest bp 21 Jul 1872 St Marks
Ann Mylchreest then married Richard Gelling 1 Feb 1876 Malew. Ann Cain should be single on census day April 2 1871.

If you look carefully at the 1881 census that Donna provided, there is an extra stepson called "John J Mylchreest" age 12 b Malew. This is before she married John Mylchreest.

(At this point I went down a wild goosechase because there is a John Mylchreest bapt 29 Nov 1868 to John Mylchreest and Elizabeth Corkill at Arbory, and a burial for a 27yo Elizabeth Mylchreest at German 19 Nov 1868. This looked like an earlier marriage for John M with wife dying of childbirth, but when the entire family showed up in 1871 and again 1881, I had to toss this idea out.)

So who is John J Mylchreest age 12?
This is him: IGI bapt of "John Joseph CAIN" 14 Jun 1868 St Marks d/o Ann CAIN.

This now leads to a whole series of wonderful census entries that will provide 2 more generations.

1871 census at St Mark's "St Mark's Cottage" No 2, Kirk Malew RG10/5777 114 pg 7 #23
Charles CAIN, Head Wid'r 77, Stone Breaker, b Marown Isle of Man
Ann CAIN, Granddaur Unmar 23 Housekeeper, b Rushen Isle of Man
John J CAIN, Great Grandson 2, b Malew Isle of Man
James GAWNE, Boarder Unm 54 Ag Lag, b Douglas Isle of Man, Blind from [?]

1861 census at Ballacubbin, Malew RG09/4424 40 pg 2 #7
Charles CAIN, Head Mar 67 Far. 40 Ac. 1 lab, b Isle of Man Marown
Elizabeth CAIN, Wife Mar 65 Wife, b Isle of Man Malew
James GAWNE, Boarder Un 45 [occupation blank), b Isle of Man Douglas
Ann CAIN, G.Daur 12 Scholar, b Isle of Man Rushen
Elizabeth CAIN, G.Daur Un 14 Servt, b Isle of Man Rushen

1851 census at Port St Mary, Rushen HO107/2526 342 pg 44 #20
John CAIN, Head Mar 25 Ag. Labourer, b Isle of Man Malew
Ann CAIN, Wife Mar 29 Ag Labourer's Wife, b Isle of Man Rushen
Betsy CAIN, Daur U 4 at home, b Isle of Man Rushen
Ann CAIN, Daur U 2 at home, b Isle of Man Rushen
Catharine CAIN, Daur U 1 at home, b Isle of Man Rushen

1851 census at Ballachrink, Malew HO107/2526 19 pg 3 #6
Chas CAINE, Head Mar 56 Farmer of 35 acres 1 labourer, b Marown
Elizabeth CAINE, Wife Mar 53 Farmer's Wife, b Malew
Wm CAINE, Son U 25 Ag Labourer, b Malew
Margt BRIDSON, Servant U 18 House Maid, b Malew

1841 census at Balla Jaraie, Kirk Malew HO107/1464 4/23 pg 14
Chas CAIN 45 Farmer Y
Eliz CAIN 45 Y
John CAIN 15 Y
Wm CAIN 14 Y
Thos CAIN 13 Y
>Wm CLARK 25 M. Servant Y
Jane BRIDSON 15 F. Serv Y

I didn't see Ann Cain's baptism on the IGI, but her two sisters either side of her, Elizabeth and Catherine, are there with parents John CAINE and Ann GAWNE, who married 14 Nov 1846 Kirk Rushen. There is a "Martha Cain" bapt 8 Oct 1848 to those parents, and they repeated the name in 1852, so maybe they renamed the first Martha to "Ann"?
Her grandparents Charles CAIN and Elizabeth BELL married 23 Aug 1823 Malew.
I think Charles will match this IGI baptism: Charles CAIN 11 May 1794 Marown s/o Wm CAIN & Elinor MYLCHREEST
John matches a patron submission "Birth" 27 Aug 1826 St Marks, s/o Charles CAINE & Elizabeth BELL.
William CAINE married Ellinor MC YLCHREEST 20 Jul 1782 Santon. I counted 9 children on the IGI 1783-1799 for Wm Caine & Ellinor Mylchreest, so I think this is the right one.

This may tie into one of Donna's Cain families, perhaps?