The best way to do this is for you and your aunt to hold title to the property jointly and to both be on the mortgage jointly. If that can't be done and on the basis that you have been told of the limitations of a declaration of trust, the other way to do it is for you to hold a second charge over the property to protect your share. The mortgage company would have a first charge for the amount of the mortgage debt and you would have a second charge for, presumably, half of the residual value of the property. I hope that helps. Please leave a positive rating so that I am credited for my time.
A second Security whereby the property can't be sold without the charge being redeemed. As I said it's like a second mortgage deed there to protect your interest in the property. It goes on the title deed.