What are the consequences of sharing my iCloud storage with a family member?
Not 100% sure about the first point, but I would imagine that it would be merged in as long as there is room etc.

For the second point - if she joins your shared storage plan she won't be allowed to continue her own personal 200GB plan. When you join the shared storage you are no longer able to also have your own private plan.

If for some reason you want to cancel the shared plan or she wants to leave it and is using over 5GB of storage(free plan threshold) - she will have 28 days to sign up for a new private plan that covers the amount of data she has stored.

Source: Apple