When you say fighting you need to be sure that you mean fighting and not dominance.
Dominance is very normal - rumbling, chasing, mounting, spraying urine. It’s something which they will do throughout their lives, and particularly when they are in season (which occurs approximately every two weeks)
Actual fighting is not normal and is bond breaking.
A full on fight in sows less common than in boars, but it can still happen. If there was to be a full on fight, it warrants immediate and permanent separation.
Sometimes sows in a dysfunctional relationship will bully the underpig - there can be a subtle line between normal dominance and bullying but it’s when chasing becomes so relentless that the underpig is constantly chased from hides, never allowed to rest. The underpig loses weight through being chased away from food all the time. Underpig becomes withdrawn and depressed.
The guides below explain behaviours further.
Dominance Behaviours In Guinea Pigs
A - Z of Guinea Pig Behaviours
Sows: Behaviour and female health problems (including ovarian cysts)