If you or your husband are a match there should be no medical cost to you. Your niece's insurance is responsible for the donor medical cost. This should include all pre and post transplant medical cost. I do not know of any program to help the donor family with any living expenses while the donor is recovering from the surgery. The programs I know of are for the recipient.
The hospital where your niece is planning on receiving the transplant should have a transplant social worker, and a transplant financial coordinator. I would get with them to see if they know of any programs for the donor's family. I would also go to the financial coordinator if you get any medical bills for anything related to pre and post transplant medical testing or treatment.
Before I had my son I did work as a transplant financial coordinator at an area hospital and would be glad to answer any questions you or your family may have. It's been about two years so the hospital staff will know the more up to date information, but I would be glad to lead you down the right path.
I hope your niece receives the transplant she needs. Do not be afraid to use the staff at the hospital that is what they are there for.