One option to consider would be to purchase mum's home. Mum could then invest the proceeds of sale in a Discounted Gift Trust to rid herself of the capital. The income she would receive from the trust could then be used to pay rent to the new landlord - i.e. you / your sister.
A Discounted Gift Trust would allow your mother to gift proceeds of sale away to through a Trust (normally in an Investment Bond). Mother would not be allowed to touch the gift until the inevitable happened to to her but she would be allowed to take a tax free income FOR LIFE, from the gift she has made.
The result is that on starting the trust and making the gift, a considerable portion (e.g. 50% for a healthy 70yr) of the amount put into it would be immediately classed as outside the estate for Inheritance Tax purposes.
After 7 years the whole amount falls outside the estate, and you have then avoided Inheritance Tax on complete gift. But of course your mother is still receiving the income for life which they can use towards living expenses, or perhaps to pay rent to you.
There would certain factors (e.g. health, affordability of you/your sisters etc.) and also whether you as a family like this route which need to be taken into account to confirm this is appropriate in this situation, but in the right circumstances could be suitable action.
Don't hesitate to contact me for clarification on anything.
Investment and Inheritance Tax Planner
Zurich Advice Network
e mail: email@example.com
Tel: (office) 0208 437 2500 (m) 07957 440 724
[I advise on Inheritance Tax Planning, and specialise in Discounted Gift Trusts and Investments in general]