Can you trace the solicitors who did the conveyancing to see if they still have their file (possibly not as they only have to be kept for 6 years - not including deeds obviously) to see if there is any evidence of this and whether it along with, possibly, a statutory declaration could get the LR to change it.
If the circumstances are explained the Land Registry would be able to advise as to what they would need in order for this to be done.
It could be the solicitor's error or could be the Land Registry's error.
I take it it makes a difference as to who the mother's share of the house would go to ie am guessing that her share under tenants in common would go to someonw other than your sister, in total at least.