Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
Given that your aunt appera to have had the right to stay in the property for life, I would say that you have an interest in possession trust created during your aunt's lifetime. Normally, that would mean that on her death, the value of the property should be included in her estate for Inheritance Tax purposes with that value also forming the cost for CGT purposes for you and your brother.
IIP trusts created before 22 March 2006 were potentially exempt transfers which means that the value falls out of the donor's estate after seven years. However, your aunt was a beneficiary with a life interest as well as being the settlor so I cannot see how the gift into trust would not remain as part of her death estate, one way or another. In effect, she was giving nothing away.
I don't believe that the trust will have a CGT liability unless it sells the property as opposed to you and your brother selling it. It might have to do that if there isn't enough cash to pay the IHT liability if there is one, then iAs far as I can see, the property is now owned by you and your sibling.
Take a look here, here, here and here for information on interest in possession trusts
I hope this helps but let me know if you have any further questions.
I suspect the advice was given by a lawyer who knew little or nothing about tax.
If the property had simply been given away by your aunt to the brothers but she retained the right to live there, that would have constituted a gift with reservation of benefit unless she paid a full market rent to them. The effect of a GWR is that the property remains in the estate of the owner and so IHT is not avoided. The brothers would then own a property they cannot sell with a cost for CGT purposes equal to the 1990 value as apportioned. THe GWR rules came into effect in 1986 so that may have something to do wiith the declaration of trust.
If I were you, I'd consult a tax lawyer to unravel it all. There is a contradiction in that the will says that your aunt leaves her eproperty to the brothers. lf it's in a trust, it's not hers any longer.