Another alternative would be to take out a whole of life insurance policy on your mum for the amount that she is putting in as a deposit.
So, for example, if your mum is contributing, let's say £80k as a deposit (just a random number) then the absolute maximum that the council should be able to claim is, I think, £65,750. The lower threshold limit below which there is nothing to pay for care home fees is currently £14,250 (80,000 - 14,250 = 65,750).
But the Council will also be able to claim interest on the outstanding amount, so it might be slightly higher.
So, in this particular example, a life insurance policy of £70k for your mum would pay off the maximum debt payable to the council.
You would need to work out the exact figures based on your own circumstances.
From there, you need to look at how much that might cost.
Getting a quote for whole of life insurance can be really difficult. It's usually those dodgy "Over 50s Life Insurance" companies that you see advertising on TV.
Another alternative, instead of taking out a life insurance policy, is to put the equivalent amount into an ISA, or something similar, every year.
You know that mum is going to pass away eventually and, if she does go into a care home before you or your DH get to 60, then those fees will eventually come out of her share of the house (although they will stop if she is still in a care home when either of you reach the age of 60).
Think about what you can afford to put away every month after you have paid the mortgage and other bills and what you spend normally to maintain a reasonable standard of living.
If you put this regular monthly savings into something like a stocks and shares ISA then this will grow tax free over the years.
You will then either get to the point where one of you hits 60 before your mum goes into care. At this point (unless the law changes) you know that your mum will not be charged any fees for a care home and everything you've saved can be spent on whatever you want to spend it on.
In contrast, if your mum goes into care before either of you reach 60 (or, again, if the law changes) then you know that you have already saved a lot towards the total bill that you might face in the future.
These are just a couple of suggestions off the top of my head. A qualified advisor would likely be able to provide you with the best advice.