Full month. 100 days seem inauspicious. Is better to accede to her request as it will avoid unnecessary issue later. You may suggest to come over to your house for celebration instead of travel to her house.
I think you have to ask the elderly at home as a form of respect especially if your elderly is traditional. 100 days is a no no very inauspicious meaning.
My husband and I also going for full month celeb.
Normally older generation will prefer full moon.
celebrated full month
100days celebration
full month
Mummy J