Thanks for your reply. So, I just need to wait for an e-mail from CIC providing me with a receipt number. I called CIC last week and they told me that CPC, Sydney is processing applications from August 27th.
The email should come from Do Not Reply - Ne Pas Repondre <Client-update-mise-a-jour@cic.gc.ca> when your application is received and opened (Acknowledgement of Receipt/AOR). Note that this takes time from the actual delivery date of mail/courier package to CIC/IRCC - the difference was about 3 weeks or so after Canada Post delivered my application to it being opened.
If you've ever interacted with CIC before (e.g. a work permit/study permit or something like that), then your Unique Client Identifier (UCI) should remain the same. I had a UCI from a work permit and included in my application for proof of citizenship was a copy of it listing that. When I finally got the AOR, the UCI is the same. Even if you don't include your UCI they may be able to match you to it based off your name, date of birth, etc.
Until you get that email (assuming it doesn't go to spam) your application won't be trackable. Even when you do, the data in the application status tool is only updated daily, so you may get the email acknowledging reciept on a given day and not be able to track the status on the tool that day (should appear the next day).
Since I included an email on the application form, they did not physically mail me anything (e.g. a physical letter acknowledging receipt) until my application was approved and they actually mailed the Citizenship Certificate.
Will they accept one set of supporting documents (ie a copy of the mother's Certificate of Citizenship, etc.) for two minor sibling applicants? I know you can send two applications together.
Also, does it matter if a certified copy of one of the supporting documents is "stale dated" or does that just apply to the application itself?
Based on an application rejected as incomplete from 2009 with my two siblings (only applied again this year by myself), I would recommend that you apply with a certified copy of each relevant supporting document per application, even if the applications are sent together, unless the notary fee is some barrier. If they consider the application incomplete, you will have to wait for them to send it back, get new certified copies per application, and then re-send it to Canada (time + cost), and wait for them to receive it/open it for review again.
As far as the dates of any certified copies, mine were all within 90 days of the time it arrived at CIC CPC-Sydney, but there is no mention of it on the site. The application must be received signed within 90 days of the date listed or it is stale, and the citizenship photos must be dated within six months of the application, but there is no mention of this for the certified copies of supporting documents.