I would imagine that your sister would have to pay the price for the new twin room but they'll onfirm once you give them a ring.

One of those rooms will more than likely have a 3rd adult reduction .. not normally much so it will probably cost a fair bit extra to add another room for 1 extra person. I've done it a couple of times with directholidays but in my case just added a 3rd adult to one of my two booked rooms .

Have you tried messing about on line with all the different permuations before you call, to see what is coming up as a the best price ? Another thing that is worth checking is if they will charge an amendment fee. i have added people to bookings and they dont seem to do this when you add someone, only if you do a name change.

Another thing that might be consider is pricing up for a single room for her as sometimes the single room supplement is fairly low, and then when you get there you should be able to use the rooms as you wish.

I did this one year in Turkey. Booked 2 adults and a child into a triple room (to get a kids price for a 12 year old) & booked my DD14 into a twin room as a single adult & then when we got there we had one room and the kids the t'other.

All sorted now thanks