You can wait to service the Proof of Service for two weeks as long as you indicate the proper date on which he was served.
Alternatively, you can file the documents by mail. Below is a link to the California Court website which will answer all your questions on how to file documents by mail, where to mail them to, how many copies you will need, etc. Good luck.
Just ensure that the date on which you actually serve him is accurate. You can return the proof of service (Via certified/tracked mail) to your sister and she can file it. OR you may be able to fax file it. Personally, I would not put an original proof of service in the mail to file with the court. If you are going to mail it, mail a copy.
Best of luck to you.
This response is intended to be a general statement of law, should not be relied upon as legal advice and does not create an attorney/client relationship.
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