If I'm reading this correctly, OP said nanny would be working only every *other* week which actually does make a difference legally, as DOL says that employer can define a workweek how they want as long as it is "seven consecutive 24-hour periods" (
https://www.dol.gov/whd/overtime_pay.htm). This means if your nanny is working 11 hours a day Mon-Fri one week and 0 hours the next week, and you define the workweek as starting Thursday at midnight, then she effectively works 33 hours one workweek (Mon-Wed) and 22 hours the next workweek (Thu-Fri). This is assuming you live in an area where OT is defined as >40 hours per week as opposed to >X hours per day.
Most payroll companies will have a default workweek definition of Mon-Sun or Sun-Sat, but legally, you can structure it such that you're not required to pay OT. However, it's probably not easy to find a nanny that is willing to work a schedule like this unless it pays enough to be her only job, so I wouldn't recommend it as it will breed resentment if you do this to save money in a way that impacts her financial security.