What is the ruling on my not speaking to my siblings in order to discipline them for their lack of respect towards my father or my elder brother, and is this considered an unlawful abandonment (hajr)?

The brothers who disobey their father and sever ties of kinship should be advised gently and kindly, and prayers should be made for their guidance. If they repent, then all is well. Otherwise, it is permissible to ostracize them if there is an overriding benefit, such as if ostracism leads to a reduction of evil. Otherwise, reconciliation is better than ostracism, while continuing to offer advice.