can I configure MemoryDB for Redis in sentinel mode?

'sentinel' is a High Availability feature. If you require HA, see High availability using replication groups. Part of the managed service is to manage this replication and detection of problems and replacement of nodes.

MemoryDB has HA built into it from the ground up - it is a good practice to start memoryDB with atleast one read replica for HA and quick failover if and when a primary fails. There is no logical reason to run Redis in Sentinel mode on MemoryDB, IMO.