Does my boycott of my siblings—due to their lack of religious commitment and their delay in giving me my inheritance—count as severing ties of kinship, or is it a permissible abandonment? And what should I do in this situation, knowing the Prophet’s (peace be upon him) حديث (saying) about maintaining kinship ties?

If you have fallen short in your duty towards your father, then repent to Allah and make up for what you missed by supplicating for him, seeking forgiveness for him, giving charity on his behalf, maintaining ties with his relatives through him, and honoring his friends. You must also maintain ties with your siblings, for Islamic law has commanded it. The sinful and non-sinful relative are equal in the obligation of maintaining ties. However, boycotting a disobedient relative depends on what is beneficial. If boycotting them helps bring them back to righteousness, then it is preferable. If maintaining ties with them while continuously advising them is more beneficial, then that is preferable. Your maintaining ties with them does not prevent you from taking their matter to court to obtain your rightful inheritance.