Just for balance, here's what it might look like from DB and DM's perspective.
The OP has voluntarily entered into an agreement with their DM which has resulted in her being given a hefty sum to 'enhance' her home. She has the benefit of DM's contribution to running costs but no care responsibilities as yet. DB may not know that she isn't a lot of help around the house, or with childcare, or indeed paying her way fully. He sees the OP's share of his DM's estate being protected from care fees, inheritance tax, and all the other costs that might eat into it.
Perhaps to him it seemed fair that he gets a similar amount now, adjusted by £10k to take account of the fact that DSis may in the future have greater responsibility for DM's care.
The problem is that life with DM is proving more burdensome than the OP realised, and she faces decades of it. She sees DB getting a no strings handout and understandably she resents it.
The OP's mother meanwhile is trying to be even handed with her DC, and failing miserably.
The OP's DM is not helping matters by throwing threats around. However she must be feeling very vulnerable, having thrown her lot in with a DD who apparently no longer wants her.
The OP's DM is young enough to live independently and it's time to do so. The OP and her DB need to facilitate this, given that they have benefited financially. It might mean giving back the money, and it might mean the OP has to move house (although the DM ought to help with the costs of that since she caused it to happen). But it will be worth it for the OP to have her life back.