I'm not sure I understand your question. Are you trying to listen to agencies at Edisto Island from sites away from Edisto? So you're trying to hear, for example, Edisto PD from your location in Summerville?
Edisto Beach seems to be a curious place for radio monitoring. The Edisto Beach site, 1-11, is apparently part of the Charleston County "portion" of Palmetto 800, meaning it's operated/maintained/controlled by Charleston County. This particular site has 2 licenses: WQNW340 licensed to Charleston County, and WQSB945 licensed to Colleton County. Based on my monitoring, which is incomplete, this site lists two "neighbor sites": Charleston County Simulcast 1-10 and Ravenel 1-59. Surprisingly, and this may be because I haven't monitored the site in a long time, the Walterboro site is not listed as a neighbor.
Neighbor sites don't mean a whole lot, but they can be clues to the way things are set up. It would be logical to me to think that Edisto Island agencies would occasionally be affiliated to the Walterboro site. However, I don't believe that I have ever monitored the Edisto PD talkgroup off of that site before. Looking back through my event logs from a recent trip to the Charleston area when I monitored the Ravenel site, I can confirm that the Edisto PD talkgroup is active on that site, at least sometimes. An affiliation request on the Charleston County simulcast site on the Edisto PD talkgroup was denied. The same is true for an affiliation request on that talkgroup on the Beaufort County SImulcast site, it was denied.
If you can hear the Ravenel site from Summerville, that might be your best chance and hearing Edisto PD. I'm pretty certain that you won't hear Edisto traffic on any Beaufort County sites, nor on the Charleston County simulcast. Likely Edisto agencies are only carried on the Edisto site (1-11) and perhaps occasionally on the Ravenel site. I'm not certain about the Walterboro site.
If I misunderstood your question, please restate it.
Brian