Necessity of having two instances (Reader and Writer) in Amazon Aurora
Yes, you are paying twice for the instances, but only once for the storage, because they share a single Aurora cluster storage volume (there is only one volume per cluster), which is part of why replication is nearly instantaneous. 

You can remove the reader instance, as long as you understand that this will slow down recovery if the lone remaining instance fails -- Aurora will have to spin up a new one, instead of automatically promoting the reader to become a writer, which is what it would otherwise do if the current writer fails.