I guess you have your terms mixed up, A subdomain is a completely separate domain, eg. subdomain.sample.com which will contain separate hosts - "www.subdomain.sample.com"/ ftp.subdomain.sample.com, to do this create the subdomain in Route53 and fill in the NameServer Records that you are provided with.
You can create a new A record, so app.sample.com, and use the IP address of the EC2 instance. Then do the same for the www.sample.com record using the WordPress IP address.
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