Thanks to Donna, for the 1851 Ramsey census info.

I think we have the correct Jane Costain, because her age in the 1871 Caernarvonshire census is given as 25, b.IoM, which is perfect to have been bapt 1846. Owen Jones 31 Quaryman, Margret daur aged 2. Both Owen and Margret b. Wales.

A William COSTAIN aged 59, Ramsey, was buried in Maughold on Dec 26 1870.

Ann COSTAIN, 76 yrs., Ramsey, buried Maughold 26 Feb 1889.

Henry COSTAIN aged 61 yrs., Ramsey, buried Maughold 8 Oct 1900.

Maughold parish register has marriages who look like Jane's siblings:
14 Jun 1863
William BISSET, Full age, Bachelor, Carpenter, Ramsey, [father] James Bisset, Sayer, and
Elizabeth COSTAIN, Full age, Spinster, Ramsey, [father] William Costain, Carpenter.

17 Jun 1866
Henry COSTAIN, Full age, Bachelor, Sawyer, Ramsey, [father] William Costain, Sawyer, and
Margaret MOORE, Full age, Spinster, Ramsey, [father] William Moore, Joiner.

15 Jun 1867
Christopher COLLISTER, Full age, Bachelor, Mariner, Ramsey, [father] Thomas Collister, Mariner, and
Ann COSTAIN, Full age, Spinster, Ramsey, [father] William Costain, Joiner.

29 Dec 1867
John PARRY, Full age, Bachelor, Quarryman, Lezayre, [father] Hugh Parry, Quarryman, and
Eliza COSTAIN, Full age, Spinster, Ramsey, [father] William Costain, Sawyer.

26 May 1883
Robert Frederick CARR, Full age, Widower, Stonemason, Maughold, [father] Robert Carr, Carpenter, and
Margaret COSTAIN, Full age, Spinster, Maughold, [father] William Costain, Joiner.

Sue