I'm not sure if I am right or not, but just trying to help. I think if your aunt requires to move in to a care home the money comes from her estate when the house which they share is sold eventually after your own mother's passing. Your mother is able to live in the house without any monetary demands. If the house is sold in the meantime, then any money owed to a care home would come from her share of the house sale and any subsequent payments would also need to be paid as required.
I hope this helps.
As far as the LA are concerned, the property should be entirely disregarded whilst your mother lives in it. Therefore there should be no deferred charge.
If a property is owned as tenants in common then does this offer the person living in it less or more protection?
We are t-in-c's and the assumption is that on death that person's 50% goes into a trust which, if the survivor needs residential care, cannot be touched to pay towards it. Has there been any change in the rules round this, the LAs seem to be trying to get round trusts.
The LA try and get round lifetime trusts because there was a big thing a few years ago "put your property into trusts to save care home fees". This is deprivation of assets and doesnt work. A trust under a Will where one partner has left a "life interest" in his or her half of the property or left it into a discretionary trust is reasonably safe because it is not a deprivation of assets situation.
TiC is the best way to own the property - there is more protection because each party can leave his or her half share however they like. I note you say "the assumption" is Zebo, is it an assumption or have you made wills?