Mislead on a will by step-mother or acting solicitor, or both?

Ok , this is going to be long. My grandfather dies in 2003, sole title deed holder on his property, left everything to his daughter (mu mum): however her step-mother ( widow), told my mum she had not been left a thing. Mum never had corresponsdance from the solicitor to explain the will: however 18 months later, step-mother calls and asks my mum is she would be happy with 50 %. Thinking she had nothing, mum agreed on the phone: again nothing from the solicitor to explain. Then mum gets a letter from the solicitor, think it was a deed of variation . asking her to sign, which she did: then gets a confirmation that she is now 50% owner. However, the original will said it was mums but step-mother had a life interest in the property, so could live there rent free but had to pay the upkeep ( as stated in the will). trust status would continue to be in force even if step- mother brought a new property. Trust was set up for the benefit of GD's children ( my mum). the solicitor who was dealing for it on behalf of the step-mother, seems to have used a verbal ok from the step-mother, to let her gain 50% of the property, but without informing my mum fully: has my mum been mislead? also, the property is now for sale, and the step-mother wants to dissolve the trust,, so mum and her get their 50% now..not comfortable with that either. any advice would be gratefully received. we are going to seek legal advice, but need something to go on. thanks