I wouldn't really want to live in a house belonging to my parents for years and years, wondering whether I'll ever get it on their death or be left homeless... A lot may happen between now and the parents' death (assuming they are in good health). Among other things, the parents may need care later in life, and may need to sell their second house to pay for it, leaving their daughter homeless.
Also CAT thresholds may be decreased and CAT rates increased.
It seems much better to do the transfer now, if it can be done with only smallish costs, OP's sister will then be secure.