Well done op, must have been hard for you, as clearly youbare still in shock she doesnt have any of your ethics and good nature.
but she certainly managed to derail it, as bogeyface says, and she is deliberately doing that to distract you. Its very clever. Politicians do it all the time. Watch Jeremy Paxman and see. Even he ends up shouting " answer the question!" And he is a professional interveiwer.
You are not taking an old lady's possessions, whatever gave you that idea?
Is that what your mil said to you? Or she said with a catch in her voice to dh?
She had already said she would sell her pictures to repay you. Did the suggestion that she sell her pictures immediately come up when she became liable to pay the loan?
Who suggested you take on the debt- because what was the difference really between doing it then or in 3 months?
Somehow, you took on this debt for 3 months - but used your long term debt facility to pay it- , you extended your mortgage to pay her debt and she was supposed to pay you back. Now here we are, over a year later and still she hasn't paid you back.
And when you talk to her, calculate the interest and say actually, its not just 15k, is 17 k or whatever is is. DO NOT write off the interest. Just don't. Otherwise its a big cash present to someone who doesn't even get her gc a soft toy. And quite clearly sleeps easily at night knowing she is causing you and your dh stress and worry, potentially wrecking your maternity leave and marriage and keeping her gc in a flat when he could have a garden.
Perfectstorm has a good idea- you need go and take objects to the value of the FULL amount of the debt, not just 15k, plus expenses - petrol for seeing valuers - and will be taking pictures/ objects the same day. Its no good getting upset, she got into debt and spent the 15k not caring how it could be paid back.
If she throws a tantrum or cries crocodile tears it will be to get you to back down and say " oh poor old lady, leave your pictures". Its worked so far hasn't it?
Stop paying her to come and see your ds -that little scam has hopefully finished- and if anyone mentions Christmas I would quite frankly laugh. She won't be seeing your ds will she? Let her tell her friends why. Or your dh can drive up there laden with gifts for " the poor old lady"
Talk to your dh about getting a charge on the house ( what a good idea, I didnt know you could do that) NOW. This week. Because what she dies suddenly, run over, any things ( and of course you hope not) and you have to contest the will. Please please don't let your dh try and fool you with words, eg, oh sis knows we are owed the money. We have seen how badly this has gone.
And I strongly suggest you and your dh get the status of her mortgage from the latest statement, check with the bank what will happen in 3 years, ie how much is left, and you get this charge done. Call a solicitor near her, ask how much, add that to the money she ows you and take her in there.
This may sound callous, but you can't take it with you, shrouds don't have pockets and there should be no reason at all she won't sign. Any stuff about " ooh, I don't like thinking about dying" well, no one does but its the one sure thing. Look at the mess your fil left, surely she doesn't want this for you?"
Make it clear its because if anything happens to her, you will not be at the top of the list when debts are paid. And of course, you won't be paying her debts. Make sure your dh hasn't signed anything.
And all that is just a safety net. You need the money NOW. She is playing you like a fish. And your DH. What does your DH want to do? Brush it under the carpet? He must find it hard, standing up to her, or having to support you as you stand up to her. Will she call him and complain about you? She isn't coming out of this very well, is she?