I'm afraid I'm confused about what you are going to be doing. I think you mean that:
The house is worth £140k. You and your sister own 35% of it (or £49,000 worth) between you. Your mother owns the remaining 65% (or £91,000 worth).
You want to buy your mother's £91,000 share for £70,000. Part of that £70,000 will go to your sister to buy her share of the house, and your mother will gift the remainder of the £70,000 to your sister.
Your mother will then live in the house rent free until 2014, at which point you'll move in. You will then buy her another house.
If I haven't misunderstood your plan, then it is certainly _legal_ to do that. However, a (non exhaustive) list of some of your problems might be:
Everything Robert said ^^^.
The gift to your sister might be subject to IHT if your mother does not survive seven years, but as you say her estate is below the threshold this is unlikely to be an issue.
Your mother has effectively given you more than £21,000 in 'house' rather than in 'cash' (the difference between the value of her share of the house and the amount you paid her for it). The amount may be larger than £21,000, as it seems that your sister's share of the house is being purchased with the £70k that you are transferring to your mother. However, since your mother will continue to live in the house, she will have made a "gift with reservation", which comes with tax consequences. I'll leave it to those more qualified to tell you what they might be!
If your mother needs care, or means tested benefits in general, the local authority/benefits agency might consider that she had deliberately got rid of her assets in order to avoid paying for that care. Again I'll leave it to people more qualified to tell you what the consequences might be there, but it would definitely cause hassle.
What happens if your mother wants to move / you want to sell the house? As you won't have lived in it, you won't get primary residence relief and you might be liable for CGT. If you want to sell but your mother doesn't want to move, you might have a costly battle. (Costly both in terms of finances and in terms of family relationships).
What happens to the house if you predecease your mother? (OK, so children ususally outlive their parents, but you need to think about it). Your mother's estate may be below the IHT threshold, but if the house is part of *your* estate then IHT may still be charged.
What happens if you become bankrupt/ need to sell the house for some reason?
What happens if you can't buy an appropriate house in 2014?
Have you thought of other (possibly easier and more tax efficient) ways of doing whatever it is you want to do? It's difficult to make sensible comments without knowing why you are considering the transaction - is it intended to give your mother access to capital now, to help your sister out by giving her cash now, to help you get a foot on the property ladder, or for some other reason?
Laura.