First, you need the horse's owner to provide you with a full lease contract. The contract will state that you are boarding the horse at your sister in law's and that you are responsible for feed, farrier, vet, etc. You need to get in on paper because even though the owners seem easy going, it can turn nasty really quicky.
On the insurance, I use Markel Equine Insurance. I used it when I leased a horse and I use it now that I own one. You can secure liability insurance (in case the horse causes injury or damage to someone or property), mortality which pays out the worth of the horse if it dies, and major medical insurance which covers things that aren't routine...colic surgeries, accidents, etc.
I chose Markel because they were the most affordable I could find and they allow you to do a payment plan to pay off your premium. I think my premium for the year is around $400 and I have liability, medical, and mortality. They will ask for a copy of your lease contract and they will want a brief explanation of why the horse is worth what the owners think it is work...training it has had, etc. If you end up buying the horse, then you just transfer the policy over to your name. Easy :)
Here is the email to the Markel broker I use, she is very nice and helpful:PGregorich@Markelcorp.com
I am sure she will answer any questions you have whether you use their insurance or not. :)
PM me if you have any other questions!