This looks a lot like Dutch, where you can say: 'We zitten samen te lezen'.
Since leser already means "are reading" and sitter means "are sitting". Why is "sitter" here? I was expecting this to say "We are sitting and reading together"...
It's explained in the tips and notes, but basically, it's a way of emphasising that the main verb (in this case reading) is happening right now, in this moment.
How would "vi leser sammen" translate?
"We are reading together" or "we read together." "Vi sitter og leser sammen" can only mean the former.
So what do "sitter" mean in this context?
Take a look at the tips and notes. It specifies the continuous present.
Woops, I always forget about the notes and tips. That's the trouble with the app version ^^;
Maybe that could be helpful a helpful add-on info when our answer is given a result.