My son in law was arrested and served 20 years for rape while overseas how do I get a copy of his military arrest records. I am concerned for my daughters welfare.

You can't. This is legally none of your business.

If your daughter decides to get a divorce, take other action against her husband, then her Attorney can subpoena these records. She can't just have access to them.

What is he doing that is worrying you?

Agreed, you will not get a copy of it, So what are you worried about, did he already do the 20 years and is out of prison now ? Did she meet him after he got out, or has she been waiting on him 20 years ?