well... she is baby "sitting" on the couch!
I would not expect her to clean the house. (sure she should clean up "her" mess/stuff) but It is not a given, that a babysitter does that and is also a household maid... unless it is succinctly specified, beforehand, and part of your "requirements" and payment.
She perhaps does not know what to do with kids nor has experience with kids. She is there watching your kids, she is a relative... I assume she was the "cheapest" sort of babysitter you could afford, though you said you are paying her well.
But unless you state your expectations/requirements... then well, she can't be expected to clean/babysit/do housework.
As it is, she is watching the kids, assisting them if they need help with something, feeding them, knows their comings and goings.... and feeds them. The basics.
But for "safety" sake... I would make SURE, she is where the kids are and gets off her butt... and "Supervises" them, especially if they are outside playing! SAFETY.
AND... your kids "make sure they have a friend's house to go to... " when their "boring" Sitter is coming over... so then, how do your kids get to their friend's house? Does she drive them? Do they walk over themselves? WHO is watching them and WHO's house are they going to.... and do you give them permission to do that? At their friend's house.... is their Parent even home and there supervising the kids too? Can they stay there ALL day or have to come home by a certain time? Is the Sitter (your relative) keeping an eye on them, there???? Or picking them up? Or are your kids just going and coming on their own??? To me, this would be a big deal and a "safety" thing and I would want to know WHERE they are going and what they are doing and if they are going to their friend's house and staying there or going other places? And do they call the Sitter and tell her where they are???? etc. etc. etc.
Then again, I know Grandparents, who babysit their Grandkids, ALL day, EVERY day... and they do not get paid for it. And the Grandparents are not social butterflies nor know what to do with young children or have mobility/health issues themselves, so the Grandkids are inside all day, or at home, with the Grandparents, doing the same things... your kids are doing with their Sitter (your relatives kid). Which is watch TV, eat, play video games etc.
Whatever you 'expect' of your Babysitter... you need to specify it and your requirements.
No matter how old, kids do require "work".... they need to be played with... because as you said, your kids are bored, with their Sitter (your relative's kid). And you have 3 kids.... to watch. So that is not easy... for a College kid that perhaps has no experience with kids.
Your relative's kid... is "watching" your kids... "sitting".... but on a couch.
In the least, she should ALSO be watching them wherever they are, Supervising, for safety and that they don't disappear off the premises, without her knowing, for example.
If you want her to do things with the kids... (and some people need it specified for them because they can't think on their own), then make a list.... and provide money for anything you want her to do with them or places you want her to take them to.
All the best,
Susan