Looking through what I have on the families you mention, I see I have another sister for Valentine - Sarah (b 1746). The source is just the IGI, but then the only sibling of the bunch that my sources have found in the parish records is Jane. (if you have another source for any of them I'd be interested).
I also have quite a few siblings for Elizabeth: William(1774), George(1777), Samuel(1781), Michael(1782), Daniel(1785), Mary(1788), Jane(1791), Valentine(1794), and another Mary(1796).
I don't have anything much on her brother William either, and several of her siblings die young, but a few produce reasonable lineages - I can send you more details if you're interested.
Obviously I find the Sentances fascinating as well - if you'd like to see more, we have a research group - I think there's another note here on that, but the link to it is: http://groups.yahoo.com/group/Sentancefamilyresearchers/http://groups.yahoo.com/group/Sentancefamilyresearchers/ We'll let you in once you apply.
Me, I'm descended from a few other Valentines, including one born 1811 in Barrowby, and one born 1741 in Folkingham, who I believe would be your Valentine's first cousin.