@HereForItMaybe Is your brother a homeowner, or does he rent?
If he owns his own home then he still has the money as much as you do. It doesn't matter whether he put the £50K towards his house purchase or not (e.g. if he already owned his house when he received the money). He can still sell his house to pay her back. If that's the case, why isn't she asking him to do that?
If he isn't a homeowner and frittered his £50K away whilst still renting, I'd ask your mother why you think it's fair to punish you for having used your £50K wisely, to buy a house, whilst your brother who wasted his isn't being asked to pay anything back.
It seems to me that right now, everyone has a roof over their heads. Your mother doesn't actually need to move. How is it fair to expect you to give up the roof over your head just so she can live under a different one?
Either way, hold firm. The answer is no. A gift is a gift. You're not selling your house in order to give your mother £50K. You're not selling your car, you're not taking out a loan. The answer is no. Your mother can still downsize if she wants to; she'll just have to choose a less expensive postcode to downsize to.
And tell your aunt to piss off. If she feels that strongly she can give your mother £50K herself. But your finances are absolutely none of her business and she needs to butt out.