Hope this helps you, it worked for me.
{
"type": "sftp",
"sync_down_on_open": true,
"host": "<your ec2 instance hostname>",
"user": "<your username>",
"remote_path": "<your remote path>",
"connect_timeout": 30,
"sftp_flags": ["-o IdentityFile=~<path to .pem file>"]
}