[throwawaybigot]

"My kid's medical records are literally attached to my wife's medical records - I have to login with her username and password to view after visit summaries etc"

Some of these decisions are in the hands of software teams. Ask out loud in a team meeting about supporting other family structures: single parents, orphans, same-sex couples, grandparent-as-guardians, etc. "The MVP works for most of our users" isn't good enough.

It's likely that the generalized solution is overall simpler too even if you won't get to use an easy label like "mother" on the form.