OUCHIE... I took care of my neighbors horse that got ran onto a tpost , but it was in his chest. Vet had us flush it with water daily, and spray it with scarlet oil. ( makes me cringe , stuff will sting, got on my cut on my hand)
Now the horse only has a tiny scar that's really not noticed.
Hope your Charro heals well,, Sounds like Auntie needs to leave the care of the animals alone. She may just have power of attorney for medical, you should find the copy to see if she has total POA on financial etc. Maybe get grandpa to sign the horse over to you ?