@Dontneedthedramaimtooold I am going to be really straight with you.
Your Mum is 73 she could live to be 100, will your marriage survive another 27 years of living with your Mum as her care needs increase?
What would happen if you died first e.g. car accident, would your DH want to be left living with his MIL?
What you and DH need to do is be really pragmatic and go through all the worse case scenarios that can happen to any of you, DH, Mum and MIL e.g. death, stroke, heart attack, dementia. Care home fees.
You then need to really look at your finances not just now but for your retirement I.e. 30-40 years.
I hope you did Deeds of Trust or something similar with both MIL’s mortgage and before taking your Mum’s money. You need to be clear how much money she is entitled to as potentially it is a % of the property.
Finally all 4 of you need to make sure you have wills and LPAs in place.
Then get legal advice on how to undo this.
You need to sell and give Mum her money back. The stamp duty is part of the costs sunk fallacy and sadly an expensive lesson.
Mum needs a sheltered retirement flat or a residential home rather than a care home. You and DH need an home for you. Financially you need to be clear what help you can and cannot provide to Mum and MIL long term without compromising your own lives. You also need to set boundaries about what care you will provide for Mum and MIL as they decline.
Finally, your DH has to start being prepared to have difficult conversations with mum and MIL. He doesn’t get to abdicate it to you, you are a partnership and need to both own the decisions you make.
I feel for you as I know just how difficult this is but not tackling this now is just going to push it down the road when it explodes and becomes far harder to resolve.