There are several resources available to help home owners remodel to accommodate disabled persons. The US Department of Housing and Urban Development (HUD) offers grants and low-interest remodeling to disabled
homeowners in lower-income brackets. In this case, the disabled person is a family member. The reality is that grant money is minimal and there is a long waiting list, usually more than a year. So that leaves low-interest government-backed or private mortgages.
Fannie Mae offers a Home Choice mortgage to help disabled persons remodel using a low-interest rate loan. They will lend up to 115% of the property's value.
Depending on where your sister lives, there may be city or state programs to help with remodeling for disabled persons. The best way to find out about these resources is to contact your local housing office and speak with a HUD counselor.