Since I know the names of many of my ancestors, I have been using the Pilot Record Search at familysearch.org to locate the siblings of them. This works well if I have the names of the parents, a marriage date, and/or place the family lived. And it helps if the family stayed in one area over the course of many years! Thanks to dedicated volunteers, many records have been indexed. In a few cases digital images have also been provided.
And the price is affordable– there is no charge to access the records! This is one resource too valuable to pass up.