E.I.
this is ok! babies will change their sleep habits - nothing stays the same for very long.
www.askdrsears.com has good information on sleep habits and sleep solutions.
if you can - check out the other questions ive answered regarding sleep. im too tired to go into detail right now. but basically, it sounds like you are making her schedule work for your family - which is important. my son didnt go to bed before us until about a month ago - he is 18 1/2 months old. so theres nothing wrong with letting them stay up with you as long as its working for you. we chose to now give him a bedtime cuz we were getting cranky at him by 9 pm... and didnt want to keep yelling at him all night. since i never let him cry it out and we coslept, he now goes to bed almost on his own - can lay down in the crib and go right to sleep without any trouble!
good luck be patient, this too will pass.