was she always stressed about them, how there state between them? who rules who
it can be dominance that equal to them, it can be fear aggression and possession for toy, food, place or even a blanket.
noting happens out of the blue, for each thing a dog does there is a reason.
you said one of them is a dog of your mother? so this dog isn't a regular in the house?
i do recommend to go for a trainer and consult, such problems needed to be addressed at home where possible to see the interactions of all with all
try to answer what i write and i'll try to help, even if not really professional to consulte before seeing the dog, anything you can try will be less stress than separation from her home and pack.
you said you keep her away, how?