OP--you said that you plan on paying for some repairs to your mom's house and giving her a holiday. Make sure that you do not give her the money for these, but actually pay for them yourselves. Arrange the holiday so that if she does cancel anything, the credit comes back to you, not in cash to mom. If she says she doesn't want a holiday/repairs and to give that money to sister/nephew, tell her that you will set the money aside in case she changes her mind in the future.
I would not respond to your sister either. If she asks again, i like the "I thought you were joking." followed by "That is not possible, all of the money has already been allocated. Bean dip." Then repeat, repeat, repeat this last phrase and don't hesitate to cut off the conversation--hang up phone, walk away, etc as needed.