charges for texting iphone to iphone when out of Canada

If WiFi is the sole means of communication to and from your phone there are no external charges for you. The same applies to your sister

mullstrom wrote:

Thank you - I just have to make sure i have it on airplane mode correct? (and data roaming off and cellular data off)? Will I still be able to text on airplane mode?

With WiFi on you will be able use SMS to reach your sister because she will be on WiFi in the same location. If you have any remaining doubts ask your carrier

When I travel I use Signal (I am abandoning WhatsApp) via WiFi too. That gets me worldwide, with voice and video calls as well

To send message while you are flying, you’ll need to use a messaging app such as WhatsApp, iMessage, .  .  .

That means you need Wi-Fi (which may require you to pay for in-flight Wi-Fi).

Ask the Cabin crew.

If you are in Mexico & don't want to use data, see this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch

Use captive Wi-Fi networks on your iPhone, iPad or iPod touch – Apple Support (UK)

For sms message you always need to use data, for social media you need Wi-Fi.

Am I charged if i send text to my sister who is at the same resort and has an iphone too?

Yes, sms message use data.