Greetings marcfromst,
Test after restarting your iPhone, as that can clear up unexpected behavior. Check out those steps here: Restart your iPhone - Apple Support
How to restart your iPhone 6, 7, 8, or SE (2nd or 3rd generation)
1. Press and hold the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo.
To isolate the behavior, see if the video is viewable after your family member sends it to a different person. If the video isn't viewable on multiple devices, then it sounds like the issue is with the sending device.
If this happens with multiple videos that affect only your iPhone, then reach out to Apple Support to help investigate the behavior. You can use the following link to reach out: Contact - Official Apple Support
Kind regards.