You may apply for a US visa whenever you wish, however: Be aware that they can deny you as a Canadian PR for the same exact reasons they would deny you as an applicant from your home country: insufficient proof of ties to your country. Being a PR of Canada does not give you any privileges of access to the US if you needed a visa before you became a PR.
Produce all the same proofs of ties to Canada as you would if you were making the application from your home country:
- letter from your employer, stating how long you have been working there, and when you are expected back at work.
- proof of residence such as lease, rental agreement or mortgage.
- proof of sufficient funds for your visit such as a bank statement.
- proof of purpose of your visit, such as an invitation letter from your sister. She may include proof she can accommodate you and support you for the period of your visit, such as her own lease or rental agreement or mortgage agreement, and her own bank statement.
- any other ties you have to Canada, such as evidence of a spouse and children (marriage and birth certificates).
It is usually advisable, if you are a recent immigrant, to wait at least a year before making the application for a US visa, because in that time you can build up all of these proofs of your ties to Canada. This is especially so if you made unsuccessful applications in the past for US visas, or if you have never applied for one before.