Kind of weird for a 6 year old, but it probably depends on the 6 year old. Maybe the biter has aggression or communication issues. Maybe he/she was having a bad day. Kids can be a bit unpredictable at times. Is it possible that he was provoked by your niece? They can mess up & do things they shouldn't be doing. My DD got bitten multiple times in daycare & they almost ALWAYS bruised, and those bites were by 1 or 2 year olds.
If it was the first time the kid had bitten someone & it was a fluke, I wouldn't make a huge deal about it, honestly. They're kids, and stuff happens. The problem with biting is that it's done on impulse, and with no warning, so it's hard to prevent. The school most likely doesn't have the resources to shadow the boy to make sure he doesn't bite again - it's a waste of the staff's time.
I'm not sure what your SIL thinks she going to gain by talking to the school. Is she going to complain every single time her kid gets hurt? I think she needs to learn how to pick her battles, personally, and teach her child how to deal with difficult kids/situations (including staying away from this kid). A one time bite? I'd let it go. Now if it became a habit, then I'd have an issue, and that would be reason to be upset.