My daughter is kind of the same way, she is very hard to get on a schedule (about a month) but it only takes a week of doing something else to totally mess that whole scheudle up. She goes to bed around 8/8:30pm, wakes up around 7:30am, and takes one nap in the afternoon for 1-2 hours.
With that what I do is pick ONE night during holidays (or if on vacation) that my daughter can stay up past her bed time (extra hour maybe two if she is not crabby tired). We still do the nap during the day, we let her sleep as long as possible so she can stay up later without being a crab. So far this has worked for us, usually it is a wedding she stays up late and dances but that is only one night and we can jump right back into the "normal" schedule the next day with little or no problems.
Remeber that you are the parents so what you say for YOUR children goes. Do not feel bad about making a good decision for your children, sleep is important and if a "strict" schedule works best for your family do not let someone mess it up. One night is fine to be a "Party Animal" and that should not mess up your scheudle too mcuh. If relative pester you about it say you would love them to stay up late but then you have to come home with me for a month and help get them back into a normal routine. That usually gets the respones, not thanks you have a good night, see you tomorrow for some more fun.
Best of luck, it is great the relatives want to spend time with your kids BUT they are your kids so do what you think is best for them in the long run.