There were probably two Catherines.
Catherine Killey would be a second marriage for both Catherine and Philip.

Brian Lawson has a burial of Catherine Brew aged 38 in Ballaugh on August 21 1847. Her maiden name was Corlett. This would be Philip's firs wife. Also she may have left a will (film number 0106447).

On February 19, 1842 in Patrick there is a marriage between Catherine Killey and James Kneale. His age is given as 31 and hers as 20.

Seven years later, on March 24 1849, Philip Brew married Catherine Kneale in Patrick . His age is given as 32, hers as 27.
Brian also has a burial of a James Kneale aged 38 in Marown on September 2 1847. If it is the same James, the dates fit.

Also there is an ancestry record on the LDS site for Margaret Brew. It appears that she married Richard Lyell Walker in 1877 in Birkenhead and moved to Australia. She died in New South Wales in 1907.