I'm quite sure OP and her husband are feeling pretty foolish for not spotting this all before and hindsight is a wonderful thing. What we need to concentrate on now is what she (and more importantly MIL) can do to sort all this out.
Does MIL work or have any income? Does she own her own home (it sounds like not if you're talking about selling yours to pay for this). Can SHE get a remortgage or a personal loan to pay you back (even if it's only for some of it)? If not, why not? Is it because she doesn't own/can't get a loan or just because she doesn't want to?
If she can get a loan of some description, the £20K needs to come straight to you, not to be paid off over time.
Otherwise, she is going to have to pay you back over time. You need to sit down with her and work out what she can afford to pay back. Even at £50 a week, it's going to be more that EIGHT YEARS before this is all paid off and you need to bear that in mind before you agree to this as opposed to any other means of repayment. It's going to be hard work for you being sweetness and light (or even just civil) while this is hanging over all of you.
As for Christmas, well to say it's cancelled is probably not too far wide of the mark is it? Because of her selfish and irresponsible behaviour, you are in so much debt thanks to her that you are having to sell your house. I guess that means that other outgoings are also going to have to be curtailed while you get this sorted. Whilst I don't think it's helpful to lay it on too thick, she needs to understand the full impact of the position that she has put you, DH and (which will probably hit home hardest) her grandchildren in.
If the worst does happen and you do have to sell the house, make sure that you add all the costs (solicitor's fees, agent's fees, removal costs) to her debt - that is actually what this is cost you, so she will also have to repay that. And make sure she does repay it, if you sell the house - selling the house is the short term solution to this to protect YOUR position, not the long term solution that she needs to deal with!
Best of luck OP.