M.G.
I agree with the other posters, however this does happen even in kids without big changes. They go through these periods sometimes. I am amazed you got her accident free at night! YOU GO WOMAN!!!! hehe Anyway, I have a three year old girl named Kaylee...she just turned 3. Anyway, she has been day potty trained for a little while already(no where near at night), but sometimes she has these spells where she wants to be the baby and will have accidents. When that happens I go to sesames streets elmos world (Yahoo search will find it asap) and I click on the first game called elmos potty time....It is real easy to use and gets the point across fast. All your daughter will have to do is press the space bar to get elmo and friends to go potty when they say they have to go. Anyway...this works for my daughter to remind her....and the fact she gets to use the comp makes her feel really big. Maybe it will work for you too. Also, the bigger deal you make of it, the worse it will be. She is still 3. Don't fight and yell with her about it...it will just make it worse.
****ALSO, if she gets busy playing with your sisters kids, she may have just forgot! It is easy to do with a lot of distractions. ELMO should be a great hit though