What is the ruling on minimizing interaction with relatives who assist oppressors and support the killing of Muslims, with clarification on the optimal way to deal with parents and siblings, and other relatives in this situation?

Endorsing the wrongful killing of people and siding with an oppressor is a grave sin, in which the one who approves shares in the iniquity with the killer and the oppressor, and it necessitates repentance.

If your relatives are doing this, and you fear they might defend falsehood, then ask them not to delve into the matter to preserve kinship ties. If they do not respond, you may limit your interaction to the bare minimum required to maintain kinship. If shunning them leads to their return (to righteousness), then it is permissible, as shunning a disobedient relative is conditional on it serving a benefit.

As for parents, it is not permissible to shun them. Rather, you should advise them gently and courteously, strive in supplicating for them, and do not speak harshly to them. A father is not like a stranger.