What the Jester said is correct in that it IS defined in the rules as written.
Having said that, if your group wanted to house rule it, that is your group's option as well. Just be aware that allowing "conjurations" to flank might open up some weird scenarios since conjurations come in many (many!) forms, so there might be some situation where it seems weird that it would flank. Plus it is one of the notable differences between a conjuration and a summoning.
And this distinction of conjurations NOT flanking becomes a 'big deal' when you have a shaman spirit companion (since they have to use powers specifically to allow their conjurations to flank as they would otherwise not be able to do so).
Having said that, if your group wanted to house rule it, that is your group's option as well. Just be aware that allowing "conjurations" to flank might open up some weird scenarios since conjurations come in many (many!) forms, so there might be some situation where it seems weird that it would flank. Plus it is one of the notable differences between a conjuration and a summoning.
And this distinction of conjurations NOT flanking becomes a 'big deal' when you have a shaman spirit companion (since they have to use powers specifically to allow their conjurations to flank as they would otherwise not be able to do so).
