To chime in to what others have posted, you really do need to give her her basic 24 hours off. That is mandated by law and you can't just exempt yourself as you have a greater need. In truth, the 24 hours off is already pretty mean (its the law and the MINIMUM you should give your helper off), but I do think if possible families should give their helpers more down time/personal time as a well rested and balanced staff makes a better one.
Put yourself in her shoes (and the truth of the matter is that it is only by the grace of God that you aren't the helper and she the employer), would you be happy if your employer cropped your already meager time off?
I also want to say that I hear this ALL the TIME. Almost every person I know thinks they are a fabulous and if not at least a fair employer, but I have said this before, the proof is in the pudding, if you have a high turnover of staff, chances are, it may not be the helpers themselves but you and your job. I have never met any person who readily admits they are a completely nasty boss even though in my assessment, the job they have on the table sounds like a nightmare. I have met people who have given their helpers days old sashimi and sushi and patted themselves on the back at their own generosity at giving the helper something they could never really afford(I am not saying this about you at all, it is an example only). Just something to think about.
Finally, I agree with Carang and think asking her honestly is the best policy. Then make some contingency plans. I find that when helpers have been treated with genuine kindness and fairness (objectively), then they will try to be accommodating to you and your family and not leave you in the lurch. However if she feels you don't have her best interests at heart and may do something that is not in her interests (i..e fire her straight up) then she's likely to do something that will at the very least severely inconvenience you.
I find that being sincerely happy for your helpers, sharing in the their joys, being fair to them and genuinely having a good relationship with them is the best ward against staff leaving you suddenly. Just like with my boss, if I genuinely believe he/she cares about me then I wouldn't want to do anything to jeopardise their livelihood but if I thought my employer was just keeping score and doing less than the minimum due to me than I would reciprocate. We are not all different really at the end of the day.