Let her know when you will be picking the girls up for YOUR weeks. If you have an enforceable ROFR, let her know that the week prior is her week, but that per the terms of the ROFR, you'd be more than happy to pick the girls up and watch them that week, also. Do not offer extra time in return. The week she is requesting is for HER benefit, not the kids'.
Then if she continues to hem and haw, might be time to start sabre-rattling regarding contempt, R3, AALs, etc.
Audaces fortuna iuvat.
The meaning of life is Happiness. What makes you happy? - Dalai Lama
For a more peaceful life, live the serenity prayer.