She chose to gift you a sum of money. That was her choice. She did so willingly and without duress. She attended a solicitor with you and signed forms to declare that the money was a gift not a loan. And you were granted your mortgage on that basis.
If she now wants the money back, HMC will consider it suspicious putting it mildly. As will your mortgage provider. You will end up with a right mess that will cost you a lot.
So when you meet with mum tomorrow, you need to firstly say you don't have £50,000. And leave it there. Don't fill the silence or offer any alternative. If she then mentions selling the house, you ask if she really wants you homeless so she can live in a posh area. And let the silence hang. And then remind her of the above. That she gifted you the money and signed documents to that effect. And that any attempt to say it wasn't a gift will probably lead to her being investigated for fraud. And that even if you did sell up the £50,000 would be used up in legal fees so plainly, it's not going to happen. Plus you would be homeless.
Remind her that you appreciated her generosity but you don't have any money to gift her and she needs to fund her relocation herself.
And there is nothing she can do. She has signed documents in front of a solicitor, who wouldn't have allowed it if she wasn't mentally fit. She and her sister can built you, but turn it back to them...
Oh mum/auntie I can't believe you would see me on the streets just so you can live in posh town! You are awful...
Please don't sell your house for your mother.