Sorry posts crossed and I don't think my link worked. As far as I understand it (although do check with jimjams' dh!) the answers are:
1)There is a difference between the capital appreciation of the house and any earning your mums and her sis make from it. Basically, when your grandad died, his estate will have been assessed for inheritance tax which is payable at a flat rate of 40% over £255,000 (I think that's the right number). So if his house was worth, say, £150,000, and he had other assets worth £100,000, there would be no tax to pay on his estate. If his house were worth £300,000, plus assets of £100,000 then you would pay tax on the amount over the tax threshold. Once the inheritance tax (if any) is paid, there is no more tax liability on the people inheriting the house until they start to earn any money from it. So, if your Mum sold the house straight away for the same amount as was stated on the inheritance tax form, she would not pay any capital gains tax. If she sold it at a later date for more than the amount stated on the inheritance tax form she would be liable for capital gains tax on the difference between the two prices. (House valued at £100,000 for probate, then sold for £110,000 would mean a £10,000 capital gain). However, both she and your aunt can make a capital gain of £7000 (this used to be the figure but it may have changed) in any tax year before paying tax, so you can knock £14,000 of any capital gain before working out what her tax bill would be. If she earned any money from renting it out, that would be taxed like regular income.
2) I believe capital gains tax is a flat rate, or perhaps it is just whatever your highest tax rate is. Maybe try the Inland Revenue website to get that confirmed.
3) I'm pretty sure that you cannot carry your capital gains allowance over to following years - you are assessed for the year in which you made the gain (ie the year in which she sold the house) and can't count any previous unused allowance. What your Mum might be able to do though, is split her half with your dad (if they are still married) as there is no tax on transfers between husband and wife and then he would have his £7000 allowance to set against any profit on the house. Another thing to check is whether you are allowed to claim the cost of doing up the house (paint, labour etc) against any eventual capital gain. That would be a question for mrjimjams.
Hope this long essay helps a bit!