The phrase "to sit in" means to occupy a position or seat, typically one that is important or prestigious. It can also refer to being part of an organization or group by regularly attending meetings and participating in activities.
Go out for walks instead of sitting in front of the TV after dinner. (newkidscenter.com)
Whether it be reading our child a book instead of sitting them in front of the TV or whatever it might be. (babycenter.com)
Instead of just sitting in front of my computer tapping away for hours on end, I scheduled in playtime, like walks in the woods with these two. (kaleandchocolate.com)