I guess just by being there by phone and email to support your sister emotionally and so she has someone to talk things over with.
Knowing someone is on her side and appreciative of her doing so much even if you not there in person would be a big emotional boost I think.

Can your sis take her to a second Dr for evaluation, without the main dr knowing??? Or does ins require a referral?
She might be able to contact ins co herself and explain the situation so they will approve without dr.