Let me try to be brief. So Im playing as Ireland.
I had about 12k troops. I charged my court chaplin to fabricate claims on the kingdom of Wales, Scotland and England.
After having my claim on these 3 countries, seeing that 12k troops is superior to the 4k of wales, I decided to attack.
Note : I was in an alliance with France (11k troops), and my brother vassals who had about 9k (his troops rivals mine). So obviously I had strong allies who upon calling them would guarantee my win.
Meanwhile, wales was in alliance with England (18k troops) and Scotland (8k troops).

To display my foolishness, I declared war on Wales and didn't feel the need to call my allies (France & my brother vassals) into the war. I was ignorant. I thought, England wouldn't honor its alliance with Wales, and even if they did, I would've seiged Wales and won before their troops arrived.

My niave decision came back to hunt me. I rallied my troops, shared them into three groups, and moved them.

I got to Wales, but to my greatest surprise, English, and Scottish troops were already in Wales, waiting for me. I didn't see them because of the Fog of War. Once my troops got down from ship, they pounced on of the 3 groups. It was commanded by my sister. So I had to move the other 2 groups to go and immediately support it. But at the end of the day, my combined troops still couldn't do much. It was 12k vs more than 25k troops, so I was bound to lose.

At that point, I started hoping that my troops should do well in the battle and retreat. That would give me time to call my powerful allies into the war. Unfortunately

1. 70% of all my knights were killed

2. I lost 4k out of the 12k troops I had

3. My lovely and beautiful sister was killed

4. My son and heir was killed (this pained me beyond measurement)

Lastly

5. I was captured and was now under the mercy of the king of Wales.

The man could've tortured me, or even killed me but he released me to end the war. I paid him 1500 gold.

The letter in which he sent me really got to me. He said My DEFEAT was bound to happen..

I just saved the game and offed the laptop
I even dreamt about the defeat while I slept. And my late son.

Meanwhile I continued playing the next day..

Something dramatic happened
You know, I've always wanted to create the Britannia empire. But with the alliance of Scotland, wakes, and England, even a blind man could tell that its impossible. Well something happened

Wales is in an alliance with England and Scotland, but Scotland isn't in an alliance with engand. The king of England, pressed the claim of one of his courtiers on one of the duchys in Scotland. So England and Scotland were at war. So, since Wales was in an alliance with Scotland, and England, they obviously had to choose a side. Funnily, Wales chose to side with Scotland, so their alliance with England broke
I saw an opportunity for me to attack wakes, but decided to wait a little since they hired mercenaries for the war and their troops were now more than 15k.

But the combined forces of Scotland and Wales couldn't match England. England completely annihilated both of their forces,
Wales total troops was now 900 and mine 14k.
O boy, I just declared war and moved my troops
I seiged their capital and captured their king.
For killing my son, I tortured him and then executed him. I am currently in control of Wales and more than half of Scotland.

Meanwhile, I created an alliance with England through marriage. The thing is, my troops are currently 21k,whike England is 20k.
I'm in alliance with France (17k) and some of my powerful vassals. I want to try attacking England, since I have the clear advantage. But doing that while we're allies would put a dent to my character. I'll be seen as an alliance breaker which would spell doom for me.

So to my question, how do I break an alliance in ck3? I obviously I'm not prepared to murder my sister, who is married to the king of England. Is there any other way?