To be able to sponsor you later, She must add you as a common law spouse.
Since you two will become common law (Dec 2018) by the time she is able to get her PR (Aug, Sept 2019), she needs to add you to her application.
Hopefully, she is apply via Express Entry or other economic/skill.
If she didn't include you in her application, she can NEVER sponsor you. (even if you two decided to get married later)