Subject: Sis & Bro UAK.zip

Hope you're both doing amazingly well! I just wanted to share something I came across that I thought you might find interesting/important/fun [delete as applicable].

I've attached a file named "UAK.zip" to this email. I think it could be [something you've discussed before, a surprise, a necessary document, etc.]. I'd love for you to take a look and let me know what you think.

Looking forward to hearing your thoughts!

Dear Sis & Bro,