If you forget about claiming any main residence relief for a minute and you sell the property for £140,000 you will make a gain of £40,000 (£140,000 - £100,000). That seems like a small increase over 11 years, though there was a crash in 2008 and 2009. The first £11,100 of he gain would be tax free and you would be left with a net taxable gain of £28,900. There are two rates of CGT for property, 18% and 28%. The rate or combination of rates that you would pay would be determined by the level of your income in the tax year you sold the property. So, your CGT liability will be somewhere between £5,202 (18%) and £8,092 (28%). Take a look here for information on how to work out the precise tax you would pay.
You have two years from when you own more than one home to make an election for one or the other to be treated as your principle private residence. For a home to be treated as such, you have to spend some time there which you clearly did. However, HMRC may challenge any such claim but only after you have sold the property and made the claim for main residence relief in your tax return. If you do make the election, for the old family home to be treated as your main home, you leave any gain you may make on the November 2014 property exposed to CGT if and when you sell it. There would appear to be nothing to lose by making the election but that will depend on how much longer you own it. The longer you do the more exposed any gain on your November 2014 becomes. If HMRC refuse to given you any main residence relief for the family home, then you would not lose out on the November 2014 property at all as far as main residence relief is concerned. Take a look here for information on the main residence and CGT.
When a gift is made in circumstances such as yours and the donor continues to live there, it is called a gift with reservation of benefit. This means that if the donor dies within seven years of making the gift its value at that time forms part of their estate. Your mother died within seven years but I doubt her estate was worth more than £312,000 or £325,000 depending which tax year she died in (see here) so there would have been no Inheritance Tax to pay. The same would have applied to your father when he died, ie the 50% share of the property he owned should be included in his estate for IHT purposes.
I hope this helps but let me know if you have any further questions.