MIL issue....any insight appreciated!

FirstTimePregnancy

First of all, let me say that my MIL is 75 years old and rarely drives.  When she does drive, it is not more than 2 miles from her house.  During a visit with her on Sunday, I happened to mention that my mother (who is MUCH younger) has said she would like to babysit for our daughter 2 days out of the week....although she has a 45 minute commute to do so.  At that time, my MIL (who lives 20 mins away), just asked what we were going to do the rest of the week - which is to put her in day care, that we have already selected.  Yesterday, my husband informed me that my MIL says she is going to come to our house and babysit the other 3 days of the week!  Keep in mind that she lives 20 mins away and seems to have a fear of driving, especially in the winter here in NY.  He tried to reason with her but my MIL is very stubborn and insists this is what she is going to do. 

In addition to her advanced age (75), my MIL has not been around any infants since my husband was in diapers!  I just don't think it's a good situation all around but my husband swears there is no arguing with her.  Any suggestions/insight?  TIA!