Hi Osborn
I don't have any doubt that the two girls are sisters of Eliphaz (on the assumption that Eliphaz is also the son of John and Susannah, which I'm not sure has been proved), as there wasn't another John and Susannah, or indeed another Osborn family, having children in Thurleigh at that time, and their son John isn't old enough to be the father, and he married an Elizabeth in any case.
The only problem is the IGI, which is not complete for Thurleigh. It's all extracted up to 1812, but 1813 onwards only female baptisms are included, which is why Eliphaz is not on the IGI. You could find they had more sons between 1813 and say 1825, as there are plenty of gaps with only Eliphaz tucked in there c1816 (although I can't see any unexplained Osborns born Thurleigh in the 1851 census). But the only way to find out is to go through the microfilm at your local LDS Family History Centre
Regards
David