your elderly relative has to consent to the sale of the house effectively surrendering her right of residence, that right will be registered as a burden on the title and you can not sell it without it being removed so yes you are stuck, for her security she would probably want another right of residence in place before she surrenders the one over the house you now live in, if she surrenders it before you buy then there is no legal obligation on you to give her another right, you could do a contract but if you break it would this person be in a position to defend it?
as for tax on the sale if it is your PPR then you will have no capital gains but that i believe only applies to the house and one acre, it may depend what you are using the balance of lands for and how much land you have which would be deemed non-residential.