giving the assassins just the barest minimum benefit of the doubt, they at least knew rumors, which means they would know she and the prince have different mothers. add onto that that shirley is starting to resemble her previous self (who had royal blood) and maybe it was just enough to be a little convincing.
they still should have known what their target looks like beforehand though.
One excuse that might barely work is:
The princess is fairly secluded in general (I think it was implied as such, she's not only afraid of the wolf lord but also a bit reclusive in general) - or if that's not the case, there's that old Japanese (Asian) custom of commoners not being allowed to look at royalty. (Maybe partially to make assassination attempts harder.)
Well, why then wouldn't the one who hired the assassins give them proper descriptions? Ah, but if the assassins fail, them having good descriptions means the suspects can be limited to the subset of people who have been able to study the princess in detail. While just saying "the Princess always hides in the garden at parties" is probably enough, while making it much harder to tie back to the instigator.