Requesting answers from someone knowledgeable about PlayStation accounts

I sold my PlayStation last year, but
my younger sibling gave it back to me recently.
It still has their account logged in.
Here is my question:
If I log in with my own account, will my sibling's account be logged out?
Or can I switch back and forth between my account and my sibling's account
to use them?
That is what I am curious about.