If you're able to text or Email your relatives, take a poll to find out which devices and apps everyone is using. Look for a common trend (ie: most of the group uses Apple devices, everyone is using Whatsapp, Viber or Skype, etc.). Once you've established which platform will work for everyone, you'll have a better idea on what to use.
Some of the possibilities that might work.....
- Viber - similar to Whatsapp.... allows free voice or video calls via WiFi at no cost. If you're using cellular data, there may be a charge.
- Skype - I believe this is also free using WiFi or a wired internet connection.
- Google hangouts - if everyone has a Gmail account, this might be a good option.
- FaceTime - if the majority of the group uses iPads / iPhones, this would probably be the easiest solution.