Thanks for your question
Has your father been paying you market value rent in this existing property ? If not then although he has gifted it to you and you are liable to capital gains on its sale, it still would have been considered for Inheritance tax as he was still making use of a property he has gifted.
The same will apply with the new purcahse - unless he pays market rent to you and your sister - then even if its not in his name, it will be treated as his for Inheritance tax purposes and you again will be liable for capital gains tax on its disposal. (if this occurs prior to your father passing away at which point is just considered for Inheritance tax)
Also you cannot pay capital gains when the next property is sold, this is not a business asset so the gaina rises at the time the sale takes place on the current property