Looking at this from 2 opposite perspectives:
On the negative side, you have no concrete proof that the Killeys (John 1810, Mary 1812, William 1814, Phillip 1816, and Eleanor 1818 ) who 've been traced so far were all siblings (children of John Killey and Mary Callow.) It's possible that one or more of them doesn't belong to this family, because there's no evidence which actually links them, eg a child of one person appearing as a niece or nephew in the household of one of the others.
Having said that, and keeping that possibility at the back of your mind, the next question is this: was Ruth b 1821 also a sibling ? The extracted IGI entry you've found, Jane, suggests that Ruth's mother was Cath Callow, whereas the patron submission suggests Ruth's mother was Mary Callow. Normally, I'd dismiss the patron submission, since so many of them are pure fiction, if it weren't for the fact that Eleanor chose the name Ruth for one of her daughters 26 years later, and Ruth was a very unusual name at that time on the IOM.
So is there an error in the extracted entry for Ruth b 1821 ? Did the person who wrote it put Cath instead of Mary for the mother's name ? Or was Mary Killey nee Callow also known by the name Cath ? There's no way of knowing. I can't see a marriage for a John Kelley and Cath Callow in the IGI, and I can't see any children from a John Killey and Cath Callow, apart from Ruth in 1821. This is a good thing, and points towards the Cath Callow-mother-of-Ruth in the extracted entry as being a mistake.
What makes you have a definite idea of a birth year of 1784 for Mary Callow ? If that was her birth year, wouldn't she have been under age at her 1804 marriage, and there'd have been some reference to it on the marriage details ? I'd put the lower and upper limits on her birth year as being 1773 and 1783 (based on being too old to have a child after 1821, and being at least 21 at her marriage.)
Since there isn't a burial for a Mary Killey which looks as though it fits, she may be indexed under Kelly, or Gill, or she may have been widowed and later remarried, or she may have moved over to England to be near one of her children.
On a positive note, you've found out a lot about the Killeys who may have been siblings of your William, and there's a strong likelihood that they actually were siblings.