I've just experienced this situation.
I have 2 sibes, a male nearly 8 and female who is 7. I have only had them for around 1 year.
before they were with me they were always with others sibes at the breeders and are well socialised
I just took in a girl who was returned to her breeder as the elderly owners finally realised that they could not look after her (she is 15kg overweight) She is nearly 2 and hasn't been socialised well with other dogs and seems to have been starved of human attention.
When I introduced her to my 2 dogs they got on fine, however within a couple of days whenever
us humans are around she stalks my girl until she snaps and they fight. She is obviously jealous of the other dogs getting
any attention from us. When alone in the backyard there wasn't a problem.
As a result of this she has gone back to the breeder to learn a few manners and hopefully get used to life with other dogs
and then will come back to us. At moment were I live we are unable to seperate the dogs unless we have some inside and some out,
but with young kids it's hard.
I'm moving in a months time to a property were each dog will have their own run so at least then things will be safer till they sort out
there differences