Does no one know the answer to this?

It's probably something to do with character encoding, or passing through different systems.

If you both use Whatsapp (Android, Apple) instead of txt the problem will go away. It works over Wifi or mobile data.

Oh, OK. Odd that the system is not unified for something so universal.

Also both phones are Samsung so it can't be an OS issue. Must be the networks.

America has always had odd cell systems, though I think they're more standard these days.

You need to tell your brother to start using the Queen's English in his texts to you, rather than those silly things.