You need to include the table that you are aliassing as inv to the from clause and then JOIN the two. For example something like this will work:
select sbi.ID as a, vbi.ID as b FROM MyFirstModule.Entity as sbi, MyFirstModule.Entity_2 as vbi INNER JOIN sbi/MyFirstModule.Entity_2_Entity/MyFirstModule.Entity_2
With as simple domain model of entity and entity2 in a 1:* relationship, this will deliver duplicates of course, but no error in the report pane
Edit: As stated the query above will deliver duplicates. When you include a second attribute in the query the statement can be performed as required. See:
select sbi.ID as a, vbi.Attribute as c, vbi.ID as b FROM MyFirstModule.Entity as sbi INNER JOIN sbi/MyFirstModule.Entity_2_Entity/MyFirstModule.Entity_2 as vbi
This will provide the correct results. Without the attribute selection the interpretation of the OQL by the server is not correct and I would suggest filing a support ticket for this. In the meantime just add an attribute other than the ID to retrieve the correct results.