(quoted from post at 11:43:36 01/18/14) Not enough facts in the first post to give good advice, but as more has come out, here's my take on it.
rrlund is exactly right about the family aspect. Your relationship with sis and bro will be "irretrievably broken" if you buy the land for lots less than it is worth, no matter what they say now.
You should save it from foreclosure, and here's how. Loan the money to your folks to pay it off, and get a Note and Deed of Trust, "all proper like". Can be on a "demand" note, at nominal interest, with no payments and no due date (due on demand). So then they will still own it, subject to the debt to you.
For estate planning purposes, it would be best for them to then deed it to you and your siblings, as tenants in common, reserving a life estate for themselves. Might as well get the 5 year look-back period for Medicaid going sooner rather than later. So long as the resulting ownership is the same as it would have gone under intestacy laws (in this case, share and share alike), there are no gift tax implications, because its an advance against your inheritance exemption, which is $2 million in Washington and $5 million or so with the Feds.
So then, the 3 of you will own it, subject to the $37,000 owed to you. Sounds like you want to keep it in the family, so you then need to figure out how siblings are going to repay you to even up the ownership. If you are going to operate it, you keep the full tenants share each year, crediting their shares against the debt until its paid. If you rent it out, same deal. Or you make arrangements to buy out their shares over time. Or one of them could buy the other two out.
This stuff should be agreed upon before you do the deal, so there are no surprises, and no hard feelings.