I agree that such a large time commitment really requires agreement BEFOREHAND. You are definitely entitled to say "no" to the extra trouble. Mothers need to take care of themselves in order to take care of their families. If you do more than you want to do, that might backfire in terms of later resentment or even anger that spills out inappropriately. Or, sometimes overcommitment and neglect of the self manifests later as illness (headaches, stomach pains or other sickness). Do what feels right for you. "Good-enough mothering" does not require making yourself stressed, pressured or exhausted. Do what you can do lovingly.