I would also suggest a friendly, upbeat private message....
"Hey, cousin! I saw your adorable pic of Baby! He is so cute! I noticed he's having some problems with the straps on his seat, though. I remember how challenging that stuff was with my first, and I wanted to share this link with you, as I found it really helpful:
http://www.drmomma.org/2010/02/common-car-seat-errors.html
I hope that helps! He is so adorable and I want him as safe as possible so I can meet him soon!

"
Okay, so it's hard to know what to say... But frame it as positively as you can, share something simple and nonthreatening, and you've done all you can.

I'd also suggest removing the picture.
