Pew that's a good question, although I even think that Google couldn't answer this question right now too, until they know how the family will look like for Stadia.
I could imagine - if I think to Google One for example - that it could be possible later to share the Stadia Pro, but if this will really be implemented...nobody knows.
I would suggest you to leave feedback in the Stadia App for this, then the developers can have another look on it and that might help. ![]()
Did you buy the founder's edition btw? If so, you could give your family member the Buddy Pass later, so that he/she can enjoy 3 months Stadia Pro.
I actually got an answer from Google support. The answer is: Once an account has Stadia Pro it CANNOT be added to another account's family plan. So if anyone is thinking about this... don't do it!