There is no way to change the value of that standard descriptor. It is automatically set by ModeShape to 'true' because it merely describes that ModeShape does supports same-name-siblings (SNS) within the repository.
Now, if you do not want to use SNS in your repository, then you can either enforce that within your application or you can have ModeShape enforce it via node types. Each node type's child node definitions specify whether they allow SNS, so the absense of the 'sns' attribute on the child node definition means that SNS are not supported. Some built-in node types (like 'nt:unstructured') allow SNS, while others (like 'nt:folder' and 'nt:file') do not. You can always define and use for your own content custom node types that don't allow SNS.