My mum wont stop giving my son and daughter a baby bottle?

Hi there, i am a mum of a 4yr old boy and nearly 2yr old girl.me andmy partner have weened them off of babys bottles as advised by health profsions.However when my kids go to there nans my mothers,she insists on giving them a baby bottle saying its there only comfort as they didnt have dummies (infact they just hated dummies or pacifiers) and i shouldnt stopt hem no should my partner. I have tried stating they are too old for bottles they dont need it for comfort and she could be making future problems for their teeth etc. But she still will not listen and has even got my dad and sister bacing her up.Its really bugging me and need help please