Hypothetically, say my wife is going for a group ride that is tracked for ZwiftPower with my account and changes height and weight accordingly. Could this cause future problems for me doing races?
Her W/kg is lower than mine (~2,9 vs 3,5), but she weighs much less (63 vs 86). Would I get flagged for weight doping or could this screw up my FTP calculation and by that my categorization?
EDIT: clarified my last sentence
Of course.
Your wife needs her own account if you don´t want to screw up your data with her results.
Thanks, I’m just not much interested in tracking my own data for analysis here, just checking for consequences for occasional racing.
I know that she has to have her own account, that’s why I said hypothetically 
From Account FAQ:
Q: Can two people share an account?
A: No. Each user on Zwift must have their own membership. Because many calculations within Zwift factor in your height, weight, and fitness level, using someone else’s account will result in a poor experience for both Zwifters.
You´ll definitely get flagged for changing “your” weight from 86 to 63 and back again. So it would work if she rode with your weight setting - which will of course give her a poor result as she would have to race in your Cat. That would mean that your FTP will drop significantly. If that´s okay with you - go ahead!
Unfortunately Zwift doesn´t offer a family membership, so there´s no option to create two Zwiftpower profiles for one Zwift account.
Hmm, bummer, thanks…
Zwift seriously needs a family membership with a small discount 
I absolutely agree!!! But unfortunately this has been requested for years with no success.