Reeaaaally sorry for the delay getting back to you. I do have some info on Sylvester, mostly on his wife and children -- but next to nothing of siblings (save Frank & Elmer) or ancestors (except Peter).
Peter remains a big mystery, though the clue you found about spelling the name differently may be the break I need in his case. (I’ve been trying for a long time to determine when the variant spelling began.)
Do I think it could be the same Elmer? Maybe, but my gut feeling is that more likely your ggfather Elmer was named after an older uncle, possibly one of Peter's brothers or uncles (assuming he had some).
A story written by my grandpa Harry (1904-1978) mentions fox hunting c. 1914 with his "excitable Uncle Elmer".Also note that the name "Wes" can be short for "Sylvester" as well as "Weston". This threw me for a loop initially.
In your next post you wrote: "Seems that Elmer, Sylvester, Frank, Weston, Jemima, and Jeannie were all of the same family their parents being Peter Detwiler and Mary Ann Smith..." wow, 3 more siblings! Nice work. Where did you find this info? I'll need a hard source for my file. (I've re-bookmarked so it shouldn't take me 4 months to reply next time!)
Other tidbits I can tell you: my ggf Sylvester had 17 children; Elmer was almost certainly his brother since both are buried quite near to each other and their father, Peter, in the same cemetery in Strongstown.
Sylvester's wife's name was Pearl Stewart. I haven't been able to establish the exact link yet, but family lore is that her nephew was James "Jimmy" Stewart, the famous actor. I did comb his museum in Indiana for clues though, and it seems likely to be true.