If my parents have both recently passed and did not have a will or no one has been made executor, can my sister change locks on our house?

First, you need to get in there and care for the pets. If they are in dire need of care, call the police. Only the executor has authority to control the house and other assets in the estate. If you or your sister are the only heirs, either of you can step up to be the Administrator, or Co-Administrators. If neither of you can agree, you can both file to become Administrator, and the Register of Wills will conduct a hearing and choose which one of you should serve.