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Learn to separate Karen from her sinful nature. We should pray for the KKK, for the potential Hitlers, pimps, child abusers, embezzlers, and bullies—for their repentance from evil sentiments and actions. We should pray for the corrupt. She's not your enemy: her sinful nature is. And she's blinded to how hateful she truly acts/feels towards others. Pray that God intervenes in her life, that he drop the scales from her eyes (like he did with Paul i.e. Acts 9:17-18) and open her heart to believe (like he did with Lydia i.e. Acts 16:14), a heart that is sensible to God and that correctly perceives how hateful she's being to those around her.

A potential blind spot: be careful to avoid responding to her in any malicious way at all—with back-biting, sarcasm, insults, what I call "verbal vengeance"—because that will not help; it'll only justify her reason for being hateful towards you (assuming she still is). Everything you think, say or do, should give her absolutely no reason to talk bad about God and Christians; may everything she sees about your conversation and behavior (towards her and others) leave her with only reasons to glorify God. People won't repent until they taste that he is good; give her a taste of God's forgiveness, refusal to retaliate, and concern for her well-being even though she cares little about you (if at all).

1 Peter 2:11-12, 15, 23 (NIV)

11 Dear friends, I urge you, as foreigners and exiles, to abstain from sinful desires, which wage war against your soul. 12 Live such good lives among the pagans that, though they accuse you of doing wrong, they may see your good deeds and glorify God on the day he visits us.
     [...]
15 For it is God’s will that by doing good you should silence the ignorant talk of foolish people.
     [...]
23 When they hurled their insults at him, he did not retaliate; when he suffered, he made no threats. Instead, he entrusted himself to him who judges justly.

We can "try" to achieve justice in this world. But it's nothing compared to what God will do to them if they don't repent. It's better that a person repent of their sins, turn from their wicked ways, believe in Jesus, and be forgiven (despite them not being punished for what they did to you), than for them to be condemned by God come judgment day. It's hypocritical of you to want to be forgiven by God, but not be willing to extend that same forgiveness towards her—forgiveness and concern (through prayer or selflessly meeting her needs/being considerate of her needs in some way), which she obviously does not deserve, just like you (and all of us) do not deserve God's forgiveness, concern or care.

Romans 12:19-21 (NIV)

19 Do not take revenge, my dear friends, but leave room for God’s wrath, for it is written: “It is mine to avenge; I will repay,”[a] says the Lord. 20 On the contrary:

    “If your enemy is hungry, feed him;
     if he is thirsty, give him something to drink.
     In doing this, you will heap burning coals on his head.”[b]

21 Do not be overcome by evil, but overcome evil with good.

Footnotes:

a. Romans 12:19 Deut. 32:35
b. Romans 12:20 Prov. 25:21,22

Matthew 5:43-48 (NASB)
“You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ 44 But I say to you, love your enemies and pray for those who persecute you, 45 so that you may be sons of your Father who is in heaven; for He causes His sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. 46 For if you love those who love you, what reward do you have? Do not even the tax collectors do the same? 47 If you greet only your brothers, what more are you doing than others? Do not even the Gentiles do the same? 48 Therefore you are to be perfect, as your heavenly Father is perfect.

Ephesians 4:32 (NASB)
Be kind to one another, tender-hearted, forgiving each other, just as God in Christ also has forgiven you.

Matthew 6:14-15 (NASB)
For if you forgive others for their transgressions, your heavenly Father will also forgive you. 15 But if you do not forgive others, then your Father will not forgive your transgressions.

Matthew 18:21-22 (NASB)
Then Peter came and said to Him, “Lord, how often shall my brother sin against me and I forgive him? Up to seven times?” 22 Jesus *said to him, “I do not say to you, up to seven times, but up to seventy times seven.

I've been struggling with forgiveness as well, it's not easy forgiving and loving someone who's wronged you but as the scriptures I provided say, we must forgive and love everyone, including our enemies because it would be evil to do otherwise and Matthew 18:21-22 is not giving a limited number for how many times we can forgive a person but rather Christ is saying we must forgive them as many times as they sin against us wheather it be 7 times, 8 times, 20 times, or 77 times we must forgive them as many times as they wrong us. What you're going through sounds pretty tough... I'll pray for you, your sister, and your family and maybe even anyone else she comes in contact with but yeah I mean I imagine this is going to be a struggle to forgive her and love her like we're called but I mean we're supposed to be praying for the KKK even. We're to pray for people like that because we're all terribly wretched and totally depraved(Romans 3:23, Genesis 6:5, Romans 3:9-18 ), it's only Christ that cleanses us of our sins and it's through him that God tranforms us and takes out our heart of stone and gives us a heart of flesh (Ezekiel 36:26-27)

Romans 3:23 (NASB)
for all have sinned and fall short of the glory of God,

http://www.capitolhillbaptist.org/audio/2014/10/what-kind-of-man-is-god-looking-for-isaiah-5-6/ (The Sermon is on the story of the Prodigal Son in Luke 15)