I don't get it. Is this for your family to message you in emergency (e.g. they lose their phones) without needing credentials to their 2FA-infested apps of sorts that they're locked out of?

Are you not getting a ton of spam from this form being open to public?

I've had similar ideas but I'd probably make it something easy to remember like myname.com/message and it quizzes the user or various things that only my family would know. Things like the color of the bedsheets, which specific IKEA kallax square the cat loves to hang out in, the location in the kitchen where the rice is stored