Messages – How can I force my iMac to use iMessage NO SMS?

My wife is traveling in the United States and has her Australian iPhone (which is roaming) and an iPhone with a SIM from the United States.

I can send a message to her. I can also send a message to my daughter (in Australia).

When I try to send a message to both my daughter and my wife, it suddenly appears as SMS (for which I have to pay the roaming charges).

Until last night it worked fine.

How can I force my iMac to use iMessage NOT SMS?