Aggressive female

Question
We have 3 female guinea pigs - 2 of them have been together since they were babies (about 3 years) and one was introduced as a baby one year ago.  They have been living happily together and one in particular seemed to be quite maternal towards the younger one.  Recently this older guinea pig has started to behave aggressively towards the younger one.  She seems to bully her, constantly chasing her away and sometimes mounting her and biting her neck.  We have separated them for short periods, but each time we re-introduce them the behaviour continues.  The guinea pigs have an extremely large area - 2 large runs on grass linked together by a tunnel, and an upstairs hutch with 2 compartments.  Also, the bullying guinea pig has a number of large tumours on her stomach which she developed about six months ago, but seemed otherwise well.  The vet said there was no way to tell whether these were malignant or benign without removing them which would be too stressful.  Until recently she has seemed fine.  Could this be linked to her behaviour?  Do you have any idea why she might be behaving like this?  Is there anything we can do other than separating them?

Answer
Hi Jo
her behaviour may well be linked to her condition.
Has the older sow caused any injury to the younger one?
The mounting is just a domination thing - she is asserting her authority over the youngster.
If she IS causing real grief to the other, you will have to seperate their living area but using a wire frame.
This will mean they can still see and sniff each other without injusry.
Of you're still worried please get back to me