There can be a CGT charge on a gift of property, particularly between connected people such as siblings.
1 The gain if it was sold today would be the difference between the disposal proceeds and the value of the property when it was gifted to you and your siblings by your mother in 2003. That was both a disposal for Capital Gains Tax purposes and a potentially exempt transfer for Inheritance Tax purposes by your mother.
2 The potential gain is £80,000. £80,000 is not the CGT liability itself.
3 Notwithstanding the fact that the property would have been included in your late mother's estate at its value when she passed away, your cost for CGT purposes is £250,000, its value when it was gifted to you. It would also have been a disposal for CGT purposes, though your mother's gain would have been exempted by the main residence relief rules.
As I said at the beginning of my answer, there can be CGT on gifts or transfers of assets between individuals. The annual CGT exemption can be used in such circumstances.
You should have an accountant or tax adviser look closely at the figures to assess the potential for a division of the properties with a view to minimising CGT.
Take a look at the notes here
on the subject of a claim that can be made between the owners of jointly owned properties on a division of those properties. Where cash is involved to equalise transfers, there may be CGT to pay.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that an individual may pay is dependent on the level of their income in the tax year that the gains arise. For 2015/16, one of the following scenarios will apply:
1 If the sum of the individual's income and the net taxable gain in 2015/16 is £42,385 or less, then all the taxable gain will be charged to CGT at 18%.
2 If the individual's income alone in 2015/16 is £42,385 or more in 2015/16, then all the taxable gain will be charged to CGT at 28%.
3 If the individual's income alone in 2015/16 is less than £42,385 but greater than £42,385 when the net taxable gain is added, then part of the net taxable gain will be charged to CGT at 18% and part at 28%.
I hope this helps but let me know if you have any further questions.