Sibling Co-house owner moving

Need your advice all. So I have a pair of siblings that own a home together and both live together, for a few years now. A year or two ago it was refinanced under just sibling 1 for a 2.5% rate. Sibling 2 is still on the deed though. House was originally bought for 450k and is now estimated to be worth over 800k. Sibling 1 wants to move to another state for work and will likely not return for a few years. What would be the fairest way for them to work out mortgage payments and responsibilities? Sibling 1 has previously said they did not want to sell. Any advice would be appreciated.

Edit: grammar