Why search siblings? You're only interested in YOUR ancestor, right? WRONG!
Researching and tracking siblings, finding their marriages, children, deaths etc can provide you with answers to questions about your own ancestor.
Let's assume you have not been able to find your great great grandfather's mother's surname before marriage. You know her first name is Mary but that's it. You find great great grandpa's death record and view it in anticipation. But sadly the informant (great great grandpa's second wife) didn't provide a surname for her mother-in-law.
You can't find great great grandpa's marriage record so no help there. But - what about a sibling? Hunt for great great grandpa's youngest sister's marriage record. Look for one of his brothers' death records. Don't overlook turning any stone available to you in your hunt for your own ancestor - remember your ancestor and his siblings shared the same parents, and those parents are your next generation back.