During my pregnancy, I'd quit my job as a tutor. However, I wrote articles for my sister, who was working as a Social Media Manager and Content Creator. Now that I want to start applying for jobs, I don't know how to show these articles in my portfolio as it bears my sister's name. How do I go about it?
Writing an article (or book) in somebody else's name is called "ghostwriting".
Typically ghost writers agree under contract to remain anonymous. If you have no such agreement with your sister, and your sister will not get into any trouble if anybody learns she did not write the articles herself (and they may, once you start making it public by telling others about it), then I would just create a section of your portfolio and label it "Ghostwritten Works". Anybody hiring a writer will understand why your sister's byline is on them. And if they ask about that, explain that is the nature of ghostwriting, you let someone else take the credit (usually for pay).
I recently read an article about creating a writing portfolio which explained your situation perfectly.
It suggested using live links towards the original content to avoid any duplicate content and SEO penalties.
Perhaps, this is something to consider as I imagine the SM-agency will be pretty SEO savvy to know if you've published their content anywhere on the web.