You can include almost anything with your index using extension method, something like this (pseudo code)

public static in SortOrder (this EntryContentBase entry)

{

var nodeEntryRelations = _relationRepository.GetParents<NodeEntryRelation>(entry.ContentLink);

var primaryParent = nodeEntryRelations.FirstOrDefault(x=>x.IsPrimary);

return primaryParent.SortOrder;  

}

Then you can include that in your conventions

Fantastic, Quan Mai, thank you very much!