Transfer of equity

Myself, my sister and brother inherited our parents house and own a third each. We are tenants I common.
4 years have passed and my sister wants to buy my brother's third. A price has been agreed and solicitors contacted. I am not part of the transaction and want to retain my share.
she will then own two-thirds and I will retain my third. 
However, the solicitor has said that this is not possible and my name must come off the deeds. Is this correct?
I don't understand this as I am not part of their transaction and not receiving any money

Roberto - you can't transfer a share of the legal ownership and it always had to be dealt with as a whole. The shares you refer to relate to the beneficial ownership.
So for example if the three of you are registered as the legal owners and one sister is buying the other sister's third beneficial share, then the three of you need to transfer the whole of the legal ownership to just the two of you, namely you (one third) and the sister (two thirds)

Thanks Adam.

Still confused.

So my name does not come off the deeds?
Am I part of the transaction and liable for SDLT even though I'm not buying an extra share?

Roberto - if still confused then it's the solicitor you need to ask for a clearer understanding as it's they who are telling you that you need to "come off the deeds"
They (or HMRC) can also explain if there is any SDLT payable and if you are liable for that also
From a purely registration point of view IF the three of you are registered as the owners then the three of you will need to transfer the whole to the two of you as I explained. What the solicitor is telling you and/or whether SDLT is payavble aren't something we can advise you on