Legal guardian control for mentally challenged family member's Apple Account

My wife, her sister, and I are combined into an Apple family. We're all over the age of 60. My sister-in-law, however, is mentally challenged, she has the mind of a three-year old. My wife is her legal guardian. My question is, will Apple set things up so that my wife can have more control of her sister's account? We are willing to provide legal documentation on request.

[Re-Titled by Moderator]

Original Title: Families that reflect reality