Is it haram to never speak to a sibling ever again?

Question

To make a long story short I have an estranged relationship with my sister. We grew very much apart after our parents divorced. I don't want to get too much into detail but she did something so awful to me that I haven't spoken to her in almost 10 years. We are now both adults living thousands of miles away from each other and I'm physically, mentally and emotionally prepared to go on like this for the rest of my life. She could die tomorrow and I honestly could care less, I probably wouldn't even go to the funeral. Is this haram according to Islam?

*She is a non Muslim by the way

Answer 1

"Severing the Ties of One's Relatives" is major sin #9 in al-Dhahabi's Major Sins (pdf), and you can see the evidence listed in that chapter. The primary evidence is Qur'an 4:1, but there's also a hadith:

The person who severs the bond of kinship will not enter Paradise.
_{Sahih al-Bukhari 5984}

Ordinarily, severing ties of kinship is a big deal.

However, the Qur'an also mentions "graceful avoidance" of harmful people.

And be patient over what they say and avoid them with gracious avoidance.
_{Qur'an 73:10}

There's a lot of details about this in Medi1Saif's answer to Is it wrong to "sever ties of kinship" in the case of an abusive relative? This includes a relaxation on the condition in the case of non-Muslim relatives.

It's up to you to decide how important the awful thing is, and determine if you're being stubborn and unforgiving, or if you're being sensible and reasonable. This may need discussing with a mufti or imam.