Because JE replication causes a current copy of your environment to be available at every data node in the group, the need for frequent backups is greatly reduced. Basically, every time a change is made on the Master, that change is backed up to every Replica node currently running. The result is that for each write operation you get a real-time incremental backup to n-1 nodes, where n are the total number of data nodes (including the Master) currently running in your replication group.
For this reason, JE does not currently support formal incremental backups of replicated environments. An application based upon the DbBackup utility class can be written to allow administrators to create full backups. This is useful for creating a backup to be stored on offline media, if your data strategy calls for that level of protection.
Remember that when performing a full backup, you should obtain the backup from a node that is current. Either use the Master node itself, or use a Replica node that must acknowledge a transaction commit before the commit operation can complete on the Master.
Note that DbBackup has some functionality that is specifically useful for replicated environments. See Backing up a Replicated Application for details.