What are the implications of selling your main residence when you have gifted half?

About 10 years ago, my mother put my sister's name on the deeds of her house and wrote a Will which left me her half and assumed that her house would eventually be divided equally between her 2 children. The reason my sister's name was put on to the deeds no longer exists. During this entire time my mother has continued to live at her home alone. Does this qualify as a gift with reservation? Recently, my mother has been considering selling her home, however, she has been given conflicting advice. She has been told that she may have to give half the money from the sale to my sister who will then have to pay capital gains tax. If this is true then is she liable for the capital gains tax rate that was in place at the time the gift was made or the rate at the time the sale is completed? My mother is now feeling extremely trapped and very confused as we all thought that if she has carried on living in her home this entire time (despite the gift being made more than 7 years ago) then it is still hers and that upon her death it would be entirely liable for inheritance tax. Until recently we were only concerned that her Will might not be sufficient as it currently stands to ensure her assets are divided equally between her 2 children. Please advise as we don't know how to proceed.