J.H.
I've been a surrogate twice for two separate families. The one thing I can't stress enough get a contract signed that covers everything you can possibly think of so if anything comes up, you can refer to the contract. I know your sister is carrying for you, but you still need a contract.
I don't know what it's like for the IM (intended mother), but I know what it's like for the surrogate. If you ever want to chat feel free to message me.
*hugs*