The essence of house sitting is that the house sitter enjoys free accommodation in exchange for taking care of the home owners house and pets. Being asked to share the cost of usage of utilities for sitting positions longer than a month is not uncommon but this is not considered 'rent'. This can be agreed and negotiated before the engagement.

If a homeowner were to charge rent, no matter how little the amount, their 'home and contents insurance policy' becomes null and void. Also the residents in their home become their tenants and the homeowner is their landlord. As a landlord there many things they are legally required to be provided and put in place. The "Residential Tenancy Act" in your state has more information. Should anything go wrong and the tenant/landlord arrangements have not been set up properly, the home owners could be subject to all sorts of financial and legal issues.