I cannot shake the feeling

At the weekend my sister lost her horse, her first horse which she was very close too. I watched as he died in her arms, she was very upset and it was awfully distressing. I comforted her as much as I could, and at the time it didn't feel like it was happening. We buried him under his favourite tree where he always stood.
After that it sank in, my sister has cried everyday and I just feel her pain so much. I was fairly close to the horse too and we had some good memories. I know a lot of you will go it's just an animal, I get that, but he was part of our family. And although it has affected me, my sister is a lot worse.

But I can't help but think about when she was 10, and we took her to the box and suprised her with him, or the times I took her riding, or walking, the times I had helped her with him. It's just so hard and idk how the hell to deal with it anymore :/