Move files from EC2 windows instance in Account A to S3 bucket in Account B

Since you said that it needs to be real time, it sounds like this is an ongoing data sync, as opposed to a migration. I'm going to make some inferences. Since you can't write directly to S3 - probably due to the application expecting a windows file system and writing locally - have you considered dropping an S3 file gateway in the environment? You could share the gateway to the windows instance and it can be mounted so that it appears as a file system, then it can sync up with S3. Typically, file gateways are put on-prem, but they don't need to be.

Thank you for the follow up. Here are more details. There are a large number of files that need to be moved. It has to be real time and it has to be cross account within the same organization. Structure does need to be preserved.