Stop letting her meet her siblings. There is no need and it is not going well. Sometimes we don't know "why" and in this case it does not matter "why." It does not work, don't do it.
I have a dog in training and meet regularly with sibling owners but the dogs never meet. They see each other at a distance as they would any other dog during training exercises but are never F2F or playing together.
I have a dog in training and meet regularly with sibling owners but the dogs never meet. They see each other at a distance as they would any other dog during training exercises but are never F2F or playing together.