I have also chosen to only tell a few immediate family members. This is such a personal decision, and since there is no right or wrong way to handle it, my advice would be to do whatever you feel most comfortable doing, and if that means keeping the band to yourself, then do it, and do NOT feel guilty! Reality is, we do most of the work anyway! The band is a wonderful tool, absolutely, but this lifestyle takes discipline, commitment and hard work in order for it to be successful. Focus on what you are doing (making healthy choices, limiting portion sizes and exercising) and proudly explain just that when someone asks your "secret".
I also have a similar situation with a relative who puts a negative spin on everything and can be very condescending (I would imagine most families have at least one person like that!), so I don't blame you one bit for choosing not to share this personal information with her! We can't change the mindset or behaviors of other people, but we do have the choice to protect ourselves from being subjected to their negativity. Good luck!