Welcome @Lovepettravels and congratulations on getting your first sit 3 weeks in Switzerland sounds great 
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The premium membership comes with some Sit cancellation cover but it’s limited to sits cancelled within 14 days of the start date and capped at a daily amount which might not fully cover the cost of alternative accommodation in Switzerland so you do need to research that for yourself . Also you need to pay for the alternative accommodation upfront and then get reimbursed ( which takes a month or two ) Sit Cancellation Terms - sitter | TrustedHousesitters.com
As a new sitter on THS there are somethings to be aware of that have been discussed on these helpful threads
https://forum.trustedhousesitters.com/t/sitter-has-to-pay-veterinary-expenses-up-front
https://forum.trustedhousesitters.com/t/i-wish-i-knew-then-what-i-know-now
We joined THS last year have completed 16 sits ( mostly long sits ) and the overall experience has been great . We have not had any sits cancelled. We have had two sits curtailed when owners came back early . So it’s a good idea to always have a Plan B in mind . Often we have had either the start or end dates changed by a day - we deal with that by regular comms before the sit .