Yes, I'd get legal advice here. I wouldn't go this alone. You can check the website below to help you find an attorney.
These 2 were recommended to me by other Realtors.
I hope everything works out for you and your family.
The first thing I would do, Tuesday morning, is call a real estate attorney. Most attorneys, Carol, will give you 15 minutes free. You will certainly want to ask them about the monthly payments thing.
You can't force a seller to sell at a price, but you may have some rights that you (and I) are unaware of. Don't go this alone.
All the best,